Sensor interrogation

ABSTRACT

A method of operating a sensor system including at least one sensor for detecting an analyte gas and a control system includes electronically interrogating the sensor to determine the operational status thereof and upon determining that the operational status is non-conforming based upon one or more predetermined thresholds, the control system initiating an automated calibration of the sensor with the analyte gas or a simulant gas.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part application of U.S. patentapplication Ser. No. 13/650,613, filed Oct. 12, 2012, which claims thebenefit of U.S. Provisional Patent Application Ser. No. 61/547,245,filed Oct. 14, 2011, and U.S. Provisional Patent Application Ser. No.61/698,153, filed Sep. 7, 2012, the disclosures of which areincorporated herein by reference.

BACKGROUND

The following information is provided to assist the reader inunderstanding certain technology including, for example, the devices,systems and/or methods disclosed below and representative environmentsin which such technology may be used. The terms used herein are notintended to be limited to any particular narrow interpretation unlessclearly stated otherwise in this document. References set forth hereinmay facilitate understanding of the technology or the backgroundthereof. The disclosure of all references cited herein are incorporatedby reference.

Prudence dictates that gas detection instrumentation be tested regularlyfor functionality. It is a common practice to, for example, perform a“bump check,” or functionality check on portable gas detectioninstrumentation on a daily basis. The purpose of this test is to ensurethe functionality of the entire gas detection system, commonly referredto as an instrument. A periodic bump check or functionality check mayalso be performed on a permanent gas detection instrument to, forexample, extend the period between full calibrations. Gas detectionsystems include at least one gas sensor, electronic circuitry and apower supply to drive the sensor, interpret its response and display itsresponse to the user. The systems further include a housing to encloseand protect such components. A bump check typically includes: a)applying a gas of interest (usually the target gas or analyte gas theinstrument is intended to detect); b) collecting and interpreting thesensor response; and c) indicating to the end user the functional stateof the system (that is, whether or not the instrument is properlyfunctioning).

Such bump tests are performed regularly and, typically, daily. Bumpchecks provide a relatively high degree of assurance to the user thatthe gas detection device is working properly. The bump check exercisesall the necessary functionalities of all parts of the gas detectiondevice in the same manner necessary to detect an alarm level of ahazardous gas. In that regard, the bump check ensures that there isefficient gas delivery from the outside of the instrument, through anytransport paths (including, for example, any protection and/or diffusionmembranes) to contact the active sensor components. The bump check alsoensures that the detection aspect of the sensor itself is workingproperly and that the sensor provides the proper response function orsignal. The bump check further ensures that the sensor is properlyconnected to its associated power supply and electronic circuitry andthat the sensor signal is interpreted properly. Moreover, the bump checkensures that the indicator(s) or user interface(s) (for example, adisplay and/or an annunciation functionality) of the gas detectioninstrument is/are functioning as intended.

However, a periodic/daily bump check requirement has a number ofsignificant drawbacks. For example, such bump checks are time consuming,especially in facilities that include many gas detection systems orinstruments. The bump check also requires the use of expensive andpotentially hazardous calibration gases. Further, the bump check alsorequires a specialized gas delivery system, usually including apressurized gas bottle, a pressure reducing regulator, and tubing andadapters to correctly supply the calibration gas to the instrument. Therequirement of a specialized gas delivery system often means that theopportunity to bump check a personal gas detection device is limited inplace and time by the availability of the gas delivery equipment.

SUMMARY

In a number of aspects hereof, devices, systems and methods of testingthe operational state or functionality of a system are provided. In anumber of embodiments, the system includes at least one gas sensor fordetecting an analyte gas (for example, an electrochemical sensor or acombustible gas sensor). The gas sensor is disposed within a housing ofthe system and is in fluid connection with an inlet system of thehousing. The system further includes at least one sensor (which may bethe same as or different from the gas sensor) within the housing and influid connection with the inlet system that is responsive to at leastone driving force created, for example, in the vicinity of the inletsystem. In a number of embodiments, the driving force is created otherthan by application of the at least one analyte gas or a simulant gasthereof to the system. The response of sensor responsive to the drivingforce provides an indication of a state of a transport path between theinlet system and sensor responsive to the driving force. The response ofthe sensor responsive to the driving force is not used to calibrate thegas sensor for detecting the analyte, but to test the state of one ormore transport paths of the system. In many embodiments hereof, thesensor responsive to the driving force need not be an analytical sensor.

The driving force may, for example, be a change in the concentration ofa gas, a change in humidity, a change in temperature, a change inpressure, or a change in flow/diffusion. In a number of embodiments, thedriving force is created by exhalation of breath in the vicinity of theinlet system. Exhaled breath causes, for example, a change in flow, achange in temperature, a change in pressure, a change in humidity,and/or a change in gas concentrations, which may be measured by thesensor responsive to the driving force. The sensor may, for example, beresponsive to changes in carbon dioxide concentration or changes inoxygen associated with the application of exhaled breath. The sensorresponsive to the presence of exhaled breath may, for example, be anelectrochemical sensor or a combustible gas sensor. In a number of otherembodiments, the driving force is created by reducing or eliminatingflow or diffusion (for example, by at least partially blocking the inletsystem). In a number of such embodiments, the sensor responsive to thedriving force is a sensor which causes a reaction of oxygen. Such asensor may, for example, be an electrochemical sensor or a combustiblegas sensor.

In a number of embodiments, devices, systems and/or methods are furtherprovided to electronically interrogate the at least one gas sensor fordetecting the analyte gas to determine the operational state orfunctionality thereof. In a number of embodiments hereof, the electronicinterrogation proceeds without the application of the analyte gas or asimulant therefor to the system. For example, in the case that thesensor for detecting the analyte gas is an electrochemical sensor,electronic interrogation may include simulating the presence of theanalyte gas electronically and measuring a response of anelectrochemical sensor to the electronic simulation. In a number ofembodiments, a constant current is caused to flow between a firstworking electrode and a counter electrode of the electrochemical sensor,and the measured response is a potential difference. In a number ofembodiments, a constant potential difference is maintained between afirst working electrode and a counter electrode of the electrochemicalsensor, and the measured response is a current. The electrochemicalsensor may, for example, be an amperometric sensor.

In the case that the at least one sensor for detecting the analyte is acombustible gas sensor, an electronic interrogation test for a sensingelement of a combustible gas sensor may include measuring the reactanceor capacitance of the sensing element. The measured reactance and/orcapacitance may be related to an operational state or functionality ofthe sensing element.

In a number of embodiments, combination of a flow or transport path testhereof and a sensor electronic interrogation test hereof may reduce oreliminate the need for bump testing a sensor system as described above.Moreover, the time period between gas calibrations may be extended in anumber of embodiments hereof

In one aspect, a method is provided of testing a system having at leastone electrochemical sensor for detecting an analyte gas within a housingof the system. The housing of the system has an inlet. The at least oneelectrochemical sensor includes an electrically active working electrodein fluid connection with the inlet of the system. The method includesbiasing the electrically active working electrode at a first potentialto detect the analyte gas and biasing the electrically active workingelectrode at a second potential, different from the first potential,such that the at least one electrochemical sensor is sensitive (at thesecond potential) to a driving force created in the vicinity of theinlet to test at least one transport path of the system. The method mayfurther include creating the driving force in the vicinity of the inletof the housing of the system and measuring a response of theelectrically active working electrode to the driving force. In a numberof embodiments, the driving force includes (a) application of exhaledbreath or (b) restricting entry of molecules into the inlet. The methodmay, for example, further include returning the electrically activeworking electrode to the first potential.

In a number of embodiments, the electrically active working electrode isresponsive to carbon dioxide at the second potential. In a number ofother embodiments, the electrically active working electrode isresponsive to oxygen at the second potential.

The method may, for example, further include electronicallyinterrogating the at least one electrochemical sensor to test thefunctionality thereof to detect the analyte gas. In a number ofembodiments, the method further includes simulating the presence of theanalyte gas electronically and measuring a response of the at least oneelectrochemical sensor to the electronic simulation. A constant currentmay, for example, be caused to flow between the electrically activeworking electrode and a counter electrode of the at least oneelectrochemical sensor, and the measured response may be a potentialdifference. A constant potential difference may, for example, bemaintained between the electrically active working electrode and acounter electrode of the at least one electrochemical sensor and themeasured response is a current.

The at least one electrochemical sensor may, for example, be anamperometric sensor. In a number of embodiments, the at least oneelectrochemical sensor includes a sensor housing comprising at least oneinlet into an interior of the sensor housing, and the electricallyactive working electrode is positioned within the sensor housing. Theelectrically active working electrode may, for example, include aneletrocatalytically active material deposited upon a porous membranethrough which gas can diffuse.

In another aspect, a system for detecting an analyte gas includes asystem housing including an inlet system, an electrochemical gas sensorwithin the housing and in fluid connection with the inlet system. Theelectrochemical sensor includes a working electrode responsive to theanalyte gas. The system further includes a control system in electricalconnection with the working electrode. The control system is operativeto bias the working electrode at a first potential at which theelectrochemical gas sensor is responsive to the analyte and operative tothe working electrode at a second potential, different from the firstpotential, at which the electrochemical gas sensor is sensitive to adriving force created in the vicinity of the inlet to test at least onetransport path of the system. As described above, the driving force may,for example, include (a) application of exhaled breath or (b)restricting entry of molecules into the inlet system. In a number ofembodiments the working electrode is responsive to carbon dioxide at thesecond potential or the working electrode is responsive to oxygen at thesecond potential.

The control system may, for example, be adapted to cause or to effectelectronic interrogation of the electrochemical sensor to test thefunctionality thereof to detect the analyte gas. The control system may,for example, be adapted to simulate the presence of the analyte gaselectronically and measure a response of the electrochemical gas sensorto the electronic simulation. In a number of embodiments, the controlsystem is adapted to cause a constant current to flow between theworking electrode and a counter electrode of the electrochemical gassensor, and the measured response is a potential difference. The controlsystem may also be adapted to maintain a constant potential differencebetween the working electrode and a counter electrode of theelectrochemical sensor, and the measured response may be a current.

In a number of embodiments of the system, the electrochemical gas sensoris an amperometric sensor. As described above, the electrochemical gassensor may, for example, include a sensor housing comprising at leastone inlet into an interior of the sensor housing, and the workingelectrode may, for example, be positioned within the sensor housing. Theworking electrode may, for example, include an eletrocatalyticallyactive material deposited upon a porous membrane through which gas candiffuse.

In a further aspect, an electrochemical sensor for detecting an analytegas in an environment includes a working electrode responsive to theanalyte gas and a control system in electrical connection with theworking electrode. The control system is operative to bias the workingelectrode at a first potential at which the electrochemical sensor isresponsive to the analyte gas and to bias the working electrode at asecond potential, different from the first potential, at which thesensor is sensitive to a driving force created in the vicinity of theinlet to test at least one transport path from the environment to theworking electrode.

In another aspect, a method of operating a sensor system including atleast one sensor for detecting an analyte gas and a control systemincludes electronically interrogating the sensor to determine theoperational status thereof and, upon determining that the operationalstatus is non-conforming based upon one or more predeterminedthresholds, the control system initiating an automated calibration ofthe sensor with the analyte gas or a simulant gas. The control systemmay, for example, be in operative connection with a system including acontainer of the analyte gas or the simulant gas. The control systemmay, for example, cause or signal the system including the container torelease the analyte gas or the simulant gas for use during the automatedcalibration. The control system may, for example, be in operativeconnection with a gas generation system to generate the analyte gas orthe simulant gas for use during the automated calibration. The controlsystem may signal or cause the gas generation system to generate theanalyte gas or the simulant gas to effect the calibration. The methodmay, for example, further include changing at least one reportingparameter of the sensor after the automated calibration. In a number ofembodiments, the sensor is an electrochemical sensor or a combustiblegas sensor.

Control systems hereof may, for example, include control circuitryand/or one or more processors (for example, microprocessors). In anumber of embodiments, the electronic interrogations hereof may, forexample, be initiated by a user or by a control system hereof (eitherwith or without user intervention). In any embodiments hereof includinga control system having a computer processor, one or more actions to beeffected by the control system may, for example, be embodied in softwarestored in a memory system in communicative connection with theprocessor.

In another aspect, a method of operating a sensor system including atleast one sensor for detecting an analyte gas and a control systemincludes electronically interrogating the at least one sensor todetermine the operational status thereof and changing at least onereporting parameter of the sensor via the control system on the basis ofresults of electronically interrogating the at least one sensor. The atleast one reporting parameter may, for example, include a range, aresolution, a cross-sensitivity parameter, a set point, and/or an alarmsignal. In a number of embodiments, the sensor is an electrochemicalsensor or a combustible gas sensor.

In a further aspect, a method of operating a sensor system including afirst sensor to detect a first gas analyte, a second sensor and acontrol system includes electronically interrogating at least the firstsensor to determine the operational status thereof and, based upon theresults of the electronic interrogation, the control system switchingfrom the first sensor to the second sensor to detect the first gasanalyte. For example, the first sensor may be determined to beinoperative to detect the first gas analyte on the basis of theelectronic interrogation. In a number of embodiments, the first sensoris an electrochemical sensor and the second sensor is an electrochemicalsensor or a combustible gas sensor.

In another aspect, a method of operating a sensor system including afirst sensor to detect a first gas analyte and a control system includeselectronically interrogating the first sensor to determine theoperational status thereof and, based upon the results of the electronicinterrogation, the control system switching the first sensor fromdetecting the first gas analyte to detecting a second gas analyte thatis different from the first gas analyte. The first sensor may, forexample, be determined to be inoperative to detect the first gas analyteon the basis of the electronic interrogation, but may still be operativeto detect the second gas analyte. The first sensor may be anelectrochemical sensor, and the second sensor may be an electrochemicalsensor. The first sensor may, for example, be a combustible sensor.

In another aspect, a method of operating a sensor system including atleast a first sensor to detect a first gas analyte and a control systemincludes electronically interrogating the sensor to determine theoperational status thereof and, based upon the results of the electronicinterrogation, the control system initiating an automated maintenanceprocedure for the sensor. The first sensor may, for example, be anelectrochemical sensor including a first working electrode responsive tothe first gas analyte. The maintenance procedure may, for example,include altering a bias potential of the first working electrode. In anumber of embodiments, the maintenance procedure includes (a) alteringthe bias potential of the first working electrode to increase asensitivity of the first working electrode to the first gas analyte, (b)altering the bias potential of the first working electrode to enhance anability of the first working electrode to discriminate against a gasother than the first gas analyte, or (c) altering the bias potential ofthe first working electrode to remove contaminant from the first workingelectrode.

In another aspect, a method is provided of operating a system includingat least a first sensor to detect a first gas analyte. The first sensoris positioned within a housing which has at least one inlet. The systemfurther includes a control system. The method includes testing a stateof at least one transport path of the system by creating a driving forcein the vicinity of the at least one inlet of the housing other than byapplication of the first gas analyte or a simulant gas for the analyteand adjusting an output of the at least first sensor via the controlsystem at least in part on the basis of the results of the test of thestate of the at least one transport path. The method may, for example,include measuring the rate of change of sensor response to the drivingforce and correlating the rate of change of sensor response to acorrection factor for sensitivity of the first sensor to the gasanalyte. In a number of embodiments, a peak in the rate of change ofsensor response to the driving force is correlated to a correctionfactor for sensitivity of the first sensor to the gas analyte. The firstsensor may, for example, include an electrochemical sensor or acombustible gas sensor. Correlation to a correction factor may, forexample, include reference to a look-up table or use of an algorithm orformula.

In another aspect, a system includes at least one sensor for detectingan analyte gas and a control system. The control system is operationalto or adapted to electronically interrogate the sensor to determine theoperational status thereof. Upon a determination that the operationalstatus is non-conforming based upon one or more predeterminedthresholds, the control system initiates an automated calibration of thesensor with the analyte gas or a simulant gas. The control system may,for example, be in operative connection with a system comprising acontainer of the analyte gas or the simulant gas. The control system maysignal or cause the system including the container to release theanalyte gas or the simulant gas for use during the automatedcalibration. The control system may be in operative connection with agas generation system to generate the analyte gas or the simulant gasfor use during the automated calibration. The control system may signalor cause the gas generation system to generate the analyte gas or thesimulant gas to generate that gas to initiate a calibration. The controlsystem may, for example, be adapted to change at least one reportingparameter of the sensor after the automated calibration. In a number ofembodiments, the sensor is an electrochemical sensor or a combustiblegas sensor.

In a further aspect, a system includes at least one sensor for detectingan analyte gas and a control system. The control system is operationalto or adapted to electronically interrogate the at least one sensor todetermine the operational status thereof, and the control system isfurther adapted to change at least one reporting parameter of the sensoron the basis of results of electronically interrogating the at least onesensor. The at least one reporting parameter may, for example, be arange, a resolution, a cross-sensitivity parameter, a set point, and/oran alarm signal. The sensor may, for example, be an electrochemicalsensor or a combustible gas sensor.

In another aspect, a system includes a first sensor to detect a firstgas analyte, a second sensor and a control system. The control system isoperational to or adapted to electronically interrogate at least thefirst sensor to determine the operational status thereof and, based uponthe results of the electronic interrogation, the control system isfurther adapted to switch from the first sensor to the second sensor todetect the first gas analyte. The first sensor may be an electrochemicalsensor, and the second sensor may be an electrochemical sensor. Thefirst sensor may be a combustible gas sensor, and the second sensor maybe combustible gas sensor.

In another aspect, a system includes a first sensor to detect a firstgas analyte and a control system. The control system is operational toor adapted to electronically interrogate the first sensor to determinethe operational status thereof and, based upon the results of theelectronic interrogation, the control system is further adapted toswitch the first sensor from detecting the first gas analyte todetecting a second gas analyte that is different from the first gasanalyte. In a number of embodiments, the first sensor is anelectrochemical sensor, and the second sensor is an electrochemicalsensor. In a number of embodiments, the first sensor is a combustiblesensor.

In another aspect, a system includes at least a first sensor to detect afirst gas analyte and a control system. The control system isoperational to or is adapted to electronically interrogate the sensor todetermine the operational status thereof. Based upon the results of theelectronic interrogation, the control system is further adapted toinitiate an automated maintenance procedure for the sensor. In a numberof embodiments, the first sensor is an electrochemical sensor comprisinga first working electrode responsive to the first gas analyte. Themaintenance procedure may, for example, include altering a biaspotential of the first working electrode. In a number of embodiments,the maintenance procedure includes: (a) altering the bias potential ofthe first working electrode to increase a sensitivity of the firstworking electrode to the first gas analyte, (b) altering the biaspotential of the first working electrode to enhance an ability of thefirst working electrode to discriminate against a gas other than thefirst gas analyte, or (c) altering the bias potential of the firstworking electrode to remove a contaminant from the first workingelectrode. In a number of embodiments, the first sensor is a combustiblesensor.

In another aspect, a system includes at least a first sensor to detect afirst gas analyte positioned within a housing having at least one inlet.The system further includes a control system. The control system isoperational to or adapted to test or interrogate a state of at least onetransport path of the system by creating a driving force in the vicinityof the at least one inlet of the housing other than by application ofthe first analyte or a simulant gas for the analyte. The system isfurther adapted to adjust an output of the at least first sensor atleast in part on the basis of the results of the test of the state ofthe at least one transport path. In a number of embodiments, a rate ofchange of sensor response to the driving force is measured andcorrelated to a correction factor for sensitivity of the first sensor tothe gas analyte. A peak in the rate of change of sensor response to thedriving force may, for example, be correlated to a correction factor forsensitivity of the first sensor to the gas analyte. The first sensormay, for example, be an electrochemical sensor or a combustible gassensor.

In another aspect, a method of operating a sensor system including afirst sensor to detect a first gas analyte includes electronicallyinterrogating at least the first sensor to determine the operationalstatus thereof and, based upon the results of the electronicinterrogation, notifying the user of at least one of a need to calibratethe sensor with the first gas analyte or a simulant gas for the firstgas analyte or a need to perform maintenance other than calibration.

In another aspect, a system includes at least a first sensor to detect afirst gas analyte positioned within a housing having at least one inlet.The system further includes a control system. The control system isoperational to or adapted to electronically interrogate at least thefirst sensor to determine the operational status thereof. Based upon theresults of the electronic interrogation, the control system is furtheradapted to notify the user of at least one of a need to calibrate thesensor with the first gas analyte or a simulant gas for the first gasanalyte or a need to perform maintenance other than calibration.

In a further aspect, a method of operating a sensor system including afirst sensor to detect a first gas analyte includes electronicallyinterrogating at least the first sensor to determine the operationalstatus thereof and based upon the results of the electronicinterrogation, notifying the user of an end of life of the sensor, whichis, for example, to occur in a determined period of time.

In a further aspect, a system includes at least a first sensor to detecta first gas analyte positioned within a housing having at least oneinlet. The system further includes a control system. The control systemis operational to or adapted to electronically interrogate at least thefirst sensor to determine the operational status thereof. Based upon theresults of the electronic interrogation, the control system is furtheradapted to notify the user of an end of life of the sensor, which is,for example, to occur in a determined period of time.

In another aspect, a method of operating a system having at least onesensor for detecting an analyte gas (for example, an analyte other thanoxygen) in an ambient atmosphere and a sensor responsive to oxygenincludes providing a volume in fluid connection with the sensorresponsive to oxygen. The volume has a state, sometimes referred toherein as an open state, in which the volume is in fluid connection withthe ambient atmosphere and at least a first restricted state, in whichentry of molecules from the ambient atmosphere into the volume isrestricted as compared to the open state. The method further includesplacing the volume in the open state, subsequently placing the volume inthe first restricted state, and measuring a dynamic output of the sensorresponsive to oxygen while the volume is in the first restricted state.The dynamic output provides an indication of the status of one or moretransport paths of the system. The molecules from the ambient atmospheremay, for example, be prevented from entering the volume in the firstrestricted state.

In a number of embodiments, the system further includes a control systemto control whether the volume is in the open state or in the firstrestricted state. The control system may, for example, be activated froma position remote from the position of the system. The control systemmay, for example, be adapted to place the volume in the open state or inthe first restricted state on the basis of a predetermined programming.This may, for example, be automated (that is, without humanintervention), and may be based, for example, upon a periodic scheduleor a triggering event.

The at least one sensor for detecting an analyte gas other than oxygenmay, for example, be an electrochemical sensor and the method may, forexample, further include electronically interrogating theelectrochemical sensor. Electronic interrogation of the electrochemicalsensor may, for example, include simulating the presence of the analytegas electronically and measuring a response of the electrochemicalsensor to the electronic simulation. The electrochemical sensor may, forexample, be an amperometric sensor.

In a number of embodiments, the electrochemical sensor is also thesensor responsive to oxygen and the electrochemical sensor includes afirst working electrode responsive to the analyte gas and a secondworking electrode responsive to oxygen. The electrochemical sensor may,for example, include a sensor housing including at least one inlet intoan interior of the sensor housing. The first working electrode and thesecond working electrode may, for example, be positioned within thesensor housing.

In a number of embodiments, the sensor responsive to oxygen is anelectrochemical gas sensor. In a number of embodiments, the sensorresponsive to oxygen is a non-analytical sensor

In another aspect, a system to detect an analyte gas in an ambientatmosphere includes a sensor to detect the analyte gas, a sensorresponsive to oxygen, a volume in fluid connection with the sensorresponsive to oxygen, and a restrictor mechanism to place the volume inan open state in which the volume is in fluid connection with theambient atmosphere and in at least a first restricted state in whichentry of molecules from the ambient atmosphere into the volume isrestricted as compared to the open state. The system further includes aprocessing system to measure a dynamic output of the sensor responsiveto oxygen while the volume is in the first restricted state. The dynamicoutput provides an indication of the status of one or more transportpaths of the system. The processing system may, for example, include aprocessor such as a microprocessor and/or other electronic circuitry. Ina number of embodiments, the molecules from the ambient atmosphere areprevented from entering the volume in the first restricted state.

The system may for example include a control system in operativeconnection with the restrictor mechanism to control whether the volumeis in the open state or in the first restricted state. The controlsystem may, for example, be activated from a position remote from theposition of the system. In a number of embodiments, the control systemis adapted to place the volume in the open state or in the firstrestricted state on the basis of predetermined programming (for example,embodied in software programming).

In a number of embodiments, the system further includes at least oneelectrochemical sensor to detect the analyte gas, a system to simulatethe presence of the analyte gas electronically, and a system to measurea response of the electrochemical sensor to the electronic simulation. Aconstant current may, for example, be caused to flow between a firstworking electrode and a counter electrode of the electrochemical sensor,and the measured response may a potential difference. A constantpotential difference may, for example, be maintained between a firstworking electrode and a counter electrode of the electrochemical sensor,and the measured response may be a current.

The electrochemical sensor may, for example, be an amperometric sensor.The electrochemical sensor may, for example, also be the sensorresponsive to oxygen, and the electrochemical sensor may include a firstworking electrode responsive to the analyte gas and a second workingelectrode responsive to oxygen. In a number of embodiments, theelectrochemical sensor includes a sensor housing including at least oneinlet into an interior of the sensor housing. The first workingelectrode and the second working electrode may, for example, bepositioned within the sensor housing.

The sensor responsive to oxygen may, for example, be an electrochemicalgas sensor. In a number of embodiments, the sensor responsive to oxygenis a non-analytical sensor.

The system may, for example, further include a housing in which thesensor to detect the analyte gas and the sensor responsive to oxygen aredisposed. The housing may, for example, include an inlet, and therestrictor mechanism may be in operative connection with the inlet to atleast partially block the inlet from fluid connection with the ambientatmosphere in the first restricted state.

In a further aspect, a method of operating a system including a housinghaving an inlet and a sensor disposed within the housing, includesproviding a first state in which the inlet is in fluid connection withan ambient atmosphere and at least a second state in which entry ofmolecules from the ambient atmosphere into the volume is more restrictedcompared to the first state, placing the inlet in the first state,subsequently placing the inlet in the second state, and measuring adynamic output or response of the sensor while the inlet is in thesecond state. The dynamic output provides an indication of the status ofone or more transport paths of the system.

In another aspect, a system includes a system housing including aninlet, at least one gas sensor responsive to a first analyte gas otherthan oxygen within the system housing and in fluid connection with theinlet, and a sensor responsive to oxygen within the system housing andin fluid connection with the inlet. The sensor responsive to oxygen isformed to be chemically separate from the at least one gas sensorresponsive to the first analyte gas other than oxygen. The sensorresponsive to oxygen is responsive to a change in the concentration ofoxygen arising from creation (for example, application) of a drivingforce in the vicinity of the inlet to provide an indication of a stateof a transport path between the inlet of the system and the at least onegas sensor responsive to the first analyte gas other than oxygen. Thedriving force may, for example, include application of exhaled breath orclosing of the inlet from fluid connection with the environmentsurrounding the housing.

The sensor responsive to oxygen may, for example, include anelectrochemically active electrode responsive to oxygen. In a number ofembodiments, the sensor responsive to oxygen is non-analyticallyresponsive to oxygen (that is, the sensor is a non-analytical sensor).

The system may, for example, further include an electrochemical gassensor analytically responsive to oxygen within the housing and in fluidconnection with the inlet. The electrochemical gas sensor analyticallyresponsive to oxygen may, for example, be formed separately from the atleast one gas sensor responsive to the first analyte gas other thanoxygen and the sensor responsive to oxygen.

The system may, for example, further include a system to electronicallyinterrogate at least one gas sensor responsive to the at least oneanalyte gas other than oxygen to test the functionality thereof todetect the at least one analyte gas other than oxygen. The system mayinclude one or more systems to electronically interrogate multiplesensors of the system.

In a number of embodiments, the at least one gas sensor responsive tothe at least one analyte gas other than oxygen is an electrochemical gassensor. The system may, for example, further include a system tosimulate the presence of the analyte gas electronically in operativeconnection with the electrochemical gas sensor responsive to the atleast one analyte gas other than oxygen and a system to measure aresponse of the electrochemical gas sensor responsive to the at leastone analyte gas other than oxygen to the electronic simulation. Aconstant current may, for example, be caused to flow between a firstworking electrode and a counter electrode of the electrochemical gassensor responsive to the at least one analyte gas other than oxygen, andthe measured response may be a potential difference. A constantpotential difference may, for example, be maintained between a firstworking electrode and a counter electrode of the electrochemical gassensor responsive to the at least one analyte gas other than oxygen, andthe measured response may be a current. The electrochemical gas sensorresponsive to the at least one analyte gas other than oxygen may, forexample, be an amperometric sensor.

In another aspect, a method of fabricating a system includes providing asystem housing which includes an inlet, disposing at least one gassensor responsive to a first analyte gas other than oxygen within thesystem housing and in fluid connection with the inlet, and disposing asensor responsive to oxygen within the system housing and in fluidconnection with the inlet. The sensor responsive to oxygen is formed tobe chemically separate from the at least one gas sensor responsive tothe first analyte gas other than oxygen. The sensor responsive to oxygenis responsive to a change in the concentration of oxygen arising fromcreation of a driving force in the vicinity of the inlet to provide anindication of a state of a transport path between the inlet of thesystem and the at least one gas sensor responsive to the first analytegas other than oxygen.

In another aspect, a method of operating a system is provided whereinthe system includes a system housing including an inlet, at least onegas sensor responsive to a first analyte gas other than oxygen withinthe system housing and in fluid connection with the inlet, and a sensorresponsive to oxygen within the system housing and in fluid connectionwith the inlet. The sensor responsive to oxygen is formed to bechemically separate from the at least one gas sensor responsive to thefirst analyte gas other than oxygen. The method includes creation of adriving force in the vicinity of the inlet that causes a change in theconcentration of oxygen in the vicinity of the sensor responsive tooxygen. The associated response of the sensor responsive to oxygenprovides an indication of a state of a transport path between the inletof the system and the at least one gas sensor responsive to the firstanalyte gas other than oxygen. The driving force may, for example,include application of exhaled breath or restricting/closing the inletfrom fluid connection with the environment surrounding the housing.

As described above, the sensor responsive to oxygen may, for example,include an electrochemically active electrode responsive to oxygen. In anumber of embodiments, the sensor responsive to oxygen isnon-analytically response to oxygen.

An electrochemical gas sensor analytically responsive to oxygen may, forexample, be disposed within the housing and in fluid connection with theinlet. The electrochemical gas sensor analytically responsive to oxygenmay, for example, be formed chemically separately from the at least onegas sensor responsive to the first analyte gas other than oxygen andfrom the sensor responsive to oxygen.

The method may further include electronically interrogating the at leastone gas sensor responsive to the at least one analyte gas other thanoxygen to test the functionality thereof to detect the at least oneanalyte gas other than oxygen. Other sensors of the system may also beelectronically interrogated. In a number of embodiments, the at leastone gas sensor responsive to the at least one analyte gas other thanoxygen is an electrochemical gas sensor, and the method further includesproviding a system to simulate the presence of the analyte gaselectronically in operative connection with the electrochemical gassensor responsive to the at least one analyte gas other than oxygen andproviding a system to measure a response of the electrochemical gassensor responsive to the at least one analyte gas other than oxygen tothe electronic simulation.

In a further aspect, a system includes a first gas sensor responsive toa first analyte gas, a second gas sensor responsive to the first analytegas, and a control system adapted to operate the first gas sensor in asensing mode wherein a signal from the first gas sensor isrepresentative of a concentration of the first analyte gas measured bythe first gas sensor and in an interrogation mode wherein the first gassensor is interrogated to test the functionality of the first gassensor. The control system is also adapted to operate the second gassensor in a sensing mode (wherein a signal from the second gas sensor isrepresentative of a concentration of the first analyte gas measured bythe second gas sensor) and in an interrogation mode (wherein the secondgas sensor is interrogated to test the functionality of the second gassensor). The control system places the first gas sensor in theinterrogation mode only if the second gas sensor is in the sensing mode,and places the second gas sensor in the interrogation mode only if thefirst gas sensor is in the sensing mode. The first sensor may, forexample, be a first electrochemical sensor, and the second sensor may bea second electrochemical gas sensor. The system may, for example,further include a system to simulate the presence of the analyte gaselectronically in operative connection with the first electrochemicalgas sensor to interrogate the first electrochemical gas sensor duringthe interrogation mode and in operative connection with the secondelectrochemical gas sensor to interrogate the second electrochemical gassensor during the interrogation mode and a system to measure a responseof the first electrochemical gas sensor to the electronic simulation anda system to measure a response of the second electrochemical gas sensorto the electronic simulation.

In still a further aspect, a method of operating a sensor including afirst gas sensor responsive to a first analyte gas and a second gassensor responsive to the first analyte gas includes operating the firstgas sensor in a sensing mode (wherein a signal from the first gas sensoris representative of a concentration of the first analyte gas measuredby the first gas sensor) and in an interrogation mode (wherein the firstgas sensor is interrogated to test the functionality of the first gassensor); operating the second gas sensor in a sensing mode (wherein asignal from the second gas sensor is representative of a concentrationof the first analyte gas measured by the second gas sensor) and in aninterrogation mode (wherein the second gas sensor is interrogated totest the functionality of the second gas sensor); and placing the firstgas sensor in the interrogation mode only if the second gas sensor is inthe sensing mode and placing the second gas sensor the interrogationmode only if the first gas sensor is in the sensing mode.

In a number of embodiments, the first sensor is a first electrochemicalsensor and the second sensor is a second electrochemical gas sensor. Themethod may, for example, further include simulating the presence of theanalyte gas electronically during the interrogation mode of the firstelectrochemical gas sensor to interrogate the first electrochemical gassensor; simulating the presence of the analyte gas electronically duringthe interrogation mode of the second sensor to interrogate the secondelectrochemical gas sensor; measuring a response of the firstelectrochemical gas sensor to the electronic simulation thereof; andmeasuring a response of the second electrochemical gas sensor to theelectronic simulation thereof.

Systems and methods hereof may further include a pump in fluidconnection with the inlet or inlet system to pump gas from the ambientatmosphere into the housing via an inlet or inlet system. In suchsystems and methods, a system to detect a pump fault may be provided inoperative connection with the pump.

The present devices, systems and/or methods, along with the attributesand attendant advantages thereof, will best be appreciated andunderstood in view of the following detailed description taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a user exhaling in a manner that the user's exhaledbreath impinges upon an inlet of a system including a housing enclosinga sensor that is sensitive to at least one property of exhaled breath.

FIG. 2A illustrates a schematic, cross-sectional view of an embodimentof a system or instrument including at least one sensor which includes afirst working electrode sensitive or responsive to an analyte and asecond electrode sensitive or responsive to a driving force associated,for example, with the presence of exhaled breath.

FIG. 2B illustrates an enlarged side, cross-sectional view of a portionof the sensor of FIG. 2A including a housing lid in which a gas inlethole is formed to be in fluid connection with a gas diffusion space anda porous gas diffusion membrane, wherein the first working electrode andthe second working electrode are formed on or attached to an interiorside of the diffusion membrane.

FIG. 2C illustrates a bottom view of the portion of the sensorillustrated in FIG. 2B.

FIG. 3A illustrates a perspective exploded view of another embodiment ofa sensor including a first working electrode sensitive or responsive toan analyte and a second electrode sensitive or responsive to thepresence of exhaled breath, wherein the first working electrode isformed on a first diffusion membrane and the second working electrode isformed on a second diffusion membrane.

FIG. 3B illustrates a cross-sectional view of the sensor of FIG. 3Awithin an instrument or system housing.

FIG. 3C illustrates an enlarged side, cross-sectional view of a portionof the sensor of FIG. 3A including a housing lid in which two gas inletholes are formed, wherein each of the first working electrode and thesecond working electrode are in general alignment with one of the twogas inlet holes.

FIG. 3D illustrates a bottom view of the portion of the sensorillustrated in FIG. 3C.

FIG. 3E illustrates a schematic, cross-sectional view of anotherembodiment of a sensor including a first working electrode sensitive orresponsive to an analyte and a second electrode sensitive or responsiveto the presence of exhaled breath, wherein the first working electrodeis formed on a first diffusion membrane positioned in a first cell andthe second working electrode is formed on a second diffusion membranepositioned in a second cell.

FIG. 3F illustrates a side, cross-sectional view of a portion of anotherembodiment of a sensor including a housing having an inlet in the formof an extending slot and a diffusion member in fluid connection with theinlet.

FIG. 3G illustrates a top view of the sensor of FIG. 3F.

FIG. 3H illustrates a schematic view of a system or instrument includinga plurality of individual sensors in fluid connection with a plenumthrough which gas to be tested is pumped to the sensors.

FIG. 3I illustrates a schematic view of a system or instrument includinga plurality of individual sensors in fluid connection with a plenumthrough which gas diffuses to the sensors.

FIG. 3J illustrates a schematic view of a system or instrument includinga plurality of individual sensors in fluid connection with a plenum ormanifold through which gas to be tested is pumped to or diffused to thesensors, wherein at least one of the sensors is a combustible gassensor.

FIG. 4 illustrates a study of the response of the sensor of FIG. 3A,wherein the first working electrode is sensitive to hydrogen sulfide andthe second working electrode is sensitive to oxygen, when challengedwith exhaled breath, followed by a mixture of 15 vol-% oxygen and 20 ppmhydrogen sulfide, followed by nitrogen.

FIG. 5A illustrates a ribbon and a wire which may be used to form sensorelements in the systems hereof, which is adapted to measure a responseto, for example, exhaled breath to test one or more transport paths ofthe system.

FIG. 5B illustrates sensor elements hereof including a conductive ribbonand a conductive wire upon which an electrocatalytic material is coatedor immobilized.

FIG. 5C illustrates a sensor element hereof including an extendingribbon having a rectangular end member which is wider than the extendingribbon.

FIG. 5D illustrates the sensor element of FIG. 5C having anelectrocatalytic material immobilized on the end member thereof.

FIG. 5E illustrates a sensor element hereof including an extending wirehaving a spiraled section on an end thereof.

FIG. 5F the sensor element of FIG. 5E including an electrocatalyticmaterial immobilized on the spiraled section thereof.

FIG. 6 illustrates an embodiment of an interdigitated electrode systemhereof wherein a first branch of the electrode system includes a firstelectrocatalytic material and a second branch includes a secondelectrocatalytic material.

FIG. 7 illustrates an embodiment of an electrode system hereof wherein afirst electrode and a second electrode are supported upon a gas porousdisk, which is formed as an annulus.

FIG. 8A illustrates the response of a representative embodiment of asingle channel amperometric sensor hereof having a single electrodefabricated to include an electrocatalytic material that is responsive toan analyte, to exhaled breath and to nitrogen

FIG. 8B illustrates an enlarged side, cross-sectional view of a portionof another embodiment of a sensor hereof including a housing lid inwhich a gas inlet hole is formed, wherein a single, multi-purposeelectrode is in general alignment with the gas inlet hole, and whereinthe electrode is operated at different biasing potentials.

FIG. 8C illustrates data from the operation of a sensor including aworking electrode operable to detect the analyte hydrogen sulfide at afirst biasing potential and to act as a non-analytical, pseudo-electrodefor detecting a change in oxygen concentration at a second potential.

FIG. 8D illustrates data from the operation of a sensor including aworking electrode operable to detect the analyte carbon monoxide at afirst biasing potential and to act as a non-analytical, pseudo-electrodefor detecting a change in oxygen concentration at a second potential.

FIG. 9A illustrates a schematic, cross-sectional view of a portion ofanother embodiment of a system hereof in which oxygen in the atmosphereis used to test or interrogate a system via dynamic coulometricmeasurement, in which a volume in fluid connection with an oxygen sensoris in an open state wherein the sensor is in fluid connection with theambient atmosphere.

FIG. 9B illustrates a schematic, cross-sectional view of the systemportion of FIG. 9A in which the volume in fluid connection with theoxygen sensor is in a restricted or closed state wherein the sensor isnot in fluid connection with the ambient atmosphere.

FIG. 9C illustrates a system including the sensor of FIG. 2A in whichoxygen in the atmosphere is used to test or interrogate the system viadynamic coulometric measurement, and in which a volume in fluidconnection with the oxygen sensor of the system is in an open state.

FIG. 9D illustrates a schematic, cross-sectional view of the system inFIG. 9C in which the volume in fluid connection with the oxygen sensoris in a restricted or closed state.

FIG. 9E illustrates a number of potential output response curves of theoxygen channel of a system such as the system of FIGS. 9C and 9D whenthe volume in fluid connection with the oxygen sensor is in a restrictedor closed state.

FIG. 10 illustrates a decision tree diagram setting forth arepresentative embodiment of an operating mode or method of a systemhereof.

FIG. 11 illustrates an equivalent circuit used to describeelectrochemical cells.

FIG. 12 illustrates a block diagram of an embodiment of measurementcircuitry for electronic interrogation.

FIG. 13A illustrates a top view of an embodiment of a sensor includingredundant working electrode/channels for detecting a single analyte anda non-analytical working electrode which is sensitive to a driving forceto effect a flow path test hereof.

FIG. 13B illustrates a bottom view of a portion of the sensor of FIG.13A illustrating the dispersion of the electrocatalytic materials forthe first analytical working electrode, the second working analyticalelectrode and the non-analytical working electrode on a single poroussupport.

FIG. 14A illustrates a flow chart of an embodiment of a sensor/systeminterrogation process hereof.

FIG. 14B illustrates a flow chart of another embodiment of asensor/system interrogation process hereof.

FIG. 14C illustrates a flow chart of another embodiment of asensor/system interrogation process hereof.

FIG. 15A illustrates sensor response or output for a typical flow pathtest hereof in the form of an exhaled breath test as a function of time.

FIG. 15B illustrates a plot of the rate of change of the sensor responseof FIG. 15A.

FIG. 15C illustrates a plot of the peak rate or change of sensorresponse as a percentage of baseline versus an associated correctionfactor for sensor sensitivity for the data of FIGS. 15A and 14B.

FIG. 16A illustrates a flow chart of an embodiment of a pump controlprocess hereof.

FIG. 16B illustrates an embodiment of a pump check process hereof

FIG. 16C is a schematic illustration of a control system for effectingthe processes of FIGS. 16A and 16B.

DETAILED DESCRIPTION

It will be readily understood that the components of the embodiments, asgenerally described and illustrated in the figures herein, may bearranged and designed in a wide variety of different configurations inaddition to the described exemplary embodiments. Thus, the followingmore detailed description of the exemplary embodiments, as representedin the figures, is not intended to limit the scope of the embodiments,as claimed, but is merely representative of exemplary embodiments.

Reference throughout this specification to “one embodiment” or “anembodiment” (or the like) means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment. Thus, the appearance of the phrases “in oneembodiment” or “in an embodiment” or the like in various placesthroughout this specification are not necessarily all referring to thesame embodiment.

Furthermore, described features, structures, or characteristics may becombined in any suitable manner in one or more embodiments. In thefollowing description, numerous specific details are provided to give athorough understanding of the embodiments. One skilled in the relevantart will recognize, however, that the various embodiments can bepracticed without one or more of the specific details, or with othermethods, components, materials, et cetera. In other instances, wellknown structures, materials, or operations are not shown or described indetail to avoid obfuscation.

As used herein and in the appended claims, the singular forms “a,” “an”,and “the” include plural references unless the context clearly dictatesotherwise. Thus, for example, reference to “a transport path” includes aplurality of such transport paths and equivalents thereof known to thoseskilled in the art, and so forth, and reference to “the transport path”is a reference to one or more such transport paths and equivalentsthereof known to those skilled in the art, and so forth.

As, for example, illustrated schematically in FIG. 1, in a number ofembodiments, the devices, systems and/or methods hereof are operable totest transport properties of a gas detection or other system 10 viaapplication of a driving force other than an analyte gas or a simulantgas (that is, a gas simulating the analyte gas by evoking a responsefrom an analytical electrode of the system) from a container to one ormore inlets 22 or an inlet system of an enclosing housing 20 of system10. In a number of embodiments, the driving force may, for example, bethe application of exhaled breath to inlet(s) 22. Housing 20 may, forexample, include a mass transport path into an interior thereof (forexample, a diffusion path) in fluid connection with inlet 22. System 10,may, for example, include, one or more filters 24 in fluid connectionwith inlet 22 either external or internal to housing 20. The path may,for example, include or be in fluid connection with a mass transport ordiffusion member or barrier 30 (for example, a membrane through whichgas is mobile (for example, via diffusion) but through which a liquidhas limited or no mobility). Housing 20 encloses a sensor 40 which issensitive to the presence of exhaled breath. For example, sensor 40 maybe sensitive to an environmental gas (the concentration of which ischanged by the presence of exhaled breath), to a gas within exhaledbreath, to a change in humidity, to a change in temperature, to a changein pressure, to a change in flow etc. A response of sensor 40 to exhaledbreath provides a measurement of the transport properties and/orfunctionality of one or more transport paths of system 10. Filter 24may, for example, be used to filter out interferent gasses (that is,gasses other than the analyte gas to which the sensor is responsive) orto filter out inhibitors or poisons.

In a number of representative embodiments discussed herein, devices,systems and/or methods hereof decrease or eliminate the necessity tobump check a gas detection instrument with stored calibration (forexample, an analyte or a simulant) gas. Such representative embodimentsof systems, devices and/or methods may, for example, combine aninternal, electronic check or interrogation of sensor functionality,connection, and/or correction without the application of an analyte gasor a simulant therefor (as, for example, described in U.S. Pat. No.7,413,645) with a transport path test using, for example, a “secondary”sensor sensitive responsive to a driving force other than the presenceof an analyte gas or a simulant gas (for example, a drivingforce/variable change arising from the presence of exhaled human breathas described above).

Many gas detection devices, instruments or systems (for example,portable gas detection instruments) include amperometric electrochemicalgas sensors. These sensors are often referred to as “fuel cell” typesensors, which refers to a primary principle of operation. Suchelectrochemical gas sensors are typically combined or integrated into adevice, system or instrument with a battery or other power supply,appropriate electronic driving circuitry (for example, including apotentiostat), a display, and one or more alarms (or other means ofcommunicating to the user the presence of a dangerous level of harmfulor toxic gas or a condition of dangerous oxygen depletion orenrichment). The sensor, circuitry and displays are typically containedin a rugged, sealed housing. As used in connection with such aninstrument, the term “sealed” refers to protection of the sensor,circuitry, and displays from harmful environmental hazards (for example,dusts, condensing vapors, such as paints or coatings, and water and/orother liquids). However, the sealed housing must continually provide forthe efficient transfer of the target or analyte gas(es) from outside theinstrument housing into a housing of the sensor itself. Often, thisresult is accomplished with one or more porous diffusion membranes thatkeep dusts, vapors, and liquids out of the instrument housing, but allowone or more analyte gases of interest to be transported into the sensoritself. This transport is typically accomplished by gaseous diffusion orby pumping an analyte gas stream into or across the face of the sensor.

As described above, the need to bump check a gas detection system/devicewith a calibration or simulant gas from a container is decreased oreliminated by providing a sensor (for example, a secondary sensor) thatis sensitive to or responds to a driving force or variable change in thevicinity of the inlet of the system, such as, for example, the presenceof exhaled breath. In a number of embodiments, components which make asensor responsive to oxygen are provided in an amperometricelectrochemical sensor (which is functional to detect an analyte otherthan oxygen). Exhaled human breath typically includes 4 to 5volume-percent (vol-%) of carbon dioxide (CO₂) and 15.8 to 16.8 vol-%oxygen (O₂). In contrast, ambient air includes approximately 20.8 vol-%O₂ and 0.035 vol-% CO₂. Thus, when a user exhales in the vicinity of oneor more inlets into the housing of the detection system or instrument,the exhaled breath displaces the volume of gas (ambient air) within adiffusion volume in a sensor therein with the exhaled breath. A responseto the decreased concentration of oxygen in exhaled breath as comparedto ambient air may be used to test the transport properties of whatevergas transport path or mechanism may be used in the gas detection device(for example, including one or more gas diffusion membranes). The sameresult may, for example, be accomplished by incorporating, within oralong with, for example, a toxic gas, a combustible or other sensorchannel, a sensing element (which may be the same as or different fromthe sensing element for the analyte) that responds to any or allcomponents of exhaled breath. For example, a similar result may beobtained by including a sensor or sensing functionality that responds tothe increased concentration of CO₂ in exhaled breath as compared toambient air. In that regard, exhaled breath contains approximately 5 vol% CO₂, as compared to ambient air, which contains approximately 600 ppmCO₂ (0.06 vol-%). A sensor or sensing system to measure CO₂concentration may, for example, include an electrochemical sensor and/ora non-dispersive infrared sensor.

Amperometric or fuel cell-type gas sensors typically include at leasttwo electrocatalytic electrodes (an anode and a cathode), at least oneof which is a gas diffusion electrode or working electrode. The workingelectrode can be either the anode or the cathode in any given sensor.The gas diffusion electrode typically includes fine particles of anelectrocatalytic material adhered to one side of a porous orgas-permeable membrane.

The electrocatalytic side of the working electrode is in ionic contactwith the second electrode (the counter electrode, whether the anode orthe cathode) via an electrolyte (for example, a liquid electrolyte, asolid electrolyte, a quasi-solid state electrolyte or an ionic liquid).A liquid electrolyte is typically a solution of a strong electrolytesalt dissolved in a suitable solvent, such as water. An organic solventmay also be used. Quasi-solid state electrolytes can, for example,include a liquid electrolyte immobilized by a high-surface-area,high-pore-volume solid. The working electrode and the counter electrodeare also in electrical contact via an external circuit used to measurethe current that flows through the sensor.

Additionally, although by no means necessary, a third or referenceelectrode, is often included. The reference electrode is constructed ina way that its potential is relatively invariant over commonly occurringenvironmental conditions. The reference electrode serves as a fixedpoint in potential space against which the operating potential of theworking electrode may be fixed. In this way, electrochemical reactionsthat would not normally be accessible may be used to detect the analytegas of interest. This result may be accomplished via control and drivingcircuitry which may, for example, include a potentiostat.

FIGS. 2A through 2C illustrate a schematic diagram of an instrument orsystem 100 including at least one electrochemical sensor or sensorsystem 110. System 100 includes a system housing 102 including an inletor inlet system 104 which places an interior of system housing 102 influid connection with the ambient environment. In the illustratedembodiment, electrochemical sensor system 110 includes at least oneprimary sensor responsive to at least one analyte gas. System 100further includes at least one secondary sensor which is responsive to adriving force or variable change outside of system housing 102 in thevicinity of inlet 104 other than a change in concentration of theanalyte gas or a simulant gas (that is, a gas other than the analyte gasto which the primary sensor is responsive) applied to system 100 from acontainer. A system 50 for creating such a driving force or variablechange is illustrated schematically in FIG. 2A. System 50 may, forexample, change the concentration of a gas, change humidity, changetemperature, change pressure, change flow or diffusion etc. in thevicinity of system inlet 104. The secondary sensor is responsive to thedriving force created by system 50. The response of the secondary sensorto the driving force is indicative of the state of the path or transportpath between inlet 104 and the secondary sensor. In general, thetransport path is the path via which a gas is transported from outsidehousing 102 (via inlet 104) to the secondary sensor (whether by, forexample, diffusion or pumping). The transport path between inlet 104 andthe secondary sensor and the transport path between inlet 104 and theprimary sensor may, for example, be the same or similar and are exposedto generally the same conditions over the life of system 100. Thesecondary sensor may, for example, be positioned in close proximity tothe primary sensor. The response of the secondary sensor to the drivingforces provides an indication of the state of the transport betweensystem inlet 104 and the primary sensor.

In a number of representative embodiments described herein, system 50represents a person who exhales in the vicinity of inlet 104. In thecase of exhaled breath, the driving force may be any one of (or morethan one of), for example, a change in the concentration of a gas (forexample, oxygen or carbon dioxide), a change in humidity, a change intemperature, a change in pressure, or a change in flow. The secondarysensor may thus include a gas sensor, a humidity sensor, a temperaturesensor, a pressure sensor and/or a flow sensor. In the case that, forexample, the secondary sensor is a humidity sensor, a temperaturesensor, a pressure sensor or a flow sensor, system 50 need not be aperson who exhales in the vicinity of system inlet 104. System 50 may,for example, be any system or device suitable to create a change inhumidity, a change in temperature, a change in pressure, or a change inflow. The degree of change in the variable of interest may, for example,be controlled to monitor for a corresponding response of the secondarysensor. In the case of a change in temperature, system 50 may, forexample, including a heating element. In the case of a change inpressure or a change in flow, system 50 may, for example, include asmall, manually operated air pump such as a bellows.

In a number of representative embodiments hereof, the secondary sensorincludes a gas sensor responsive to the concentration of a gas which ischanged by exhalation in the vicinity of system inlet 104. In severalsuch embodiments, sensor 110 includes a housing 120 having a gas inlet130 (formed in a lid 122 of sensor housing 120) for entry of analyte gasand human breath into sensor 110. In the illustrated embodiment, inlet130 is in fluid connection with a gas diffusion volume or space 118.Electrolyte saturated wick materials 140 a, 140 b and 140 c separate afirst working electrode 150 a (responsive to the presence of analytegas) and a second working electrode 150 b (responsive to the presence ofhuman breath) from reference electrode(s) 170 and counter electrode(s)180 within sensor 110 and provide ionic conduction therebetween via theelectrolyte absorbed therein. First working electrode 150 a, referenceelectrode 170 and counter electrode 180, in cooperation with electrolytesaturated wick materials 140 a, 140 b and 140 c form a portion of theprimary sensor. Second working electrode 150 b, reference electrode 170and counter electrode 180, in cooperation with electrolyte saturatedwick materials 140 a, 140 b and 140 c form a portion of the secondarysensor. Electronic circuitry 190 as known in the art is provided, forexample, to maintain a desired potential between working electrodes 150a and 150 b and reference electrode(s) 170, to process an output signalfrom sensor 110 and to connect/communicate with other components ofsystem 100 (including, for example, one or more displays, communicationsystems, power supplies etc.).

In the illustrated embodiment, first working electrode 150 a and secondworking electrode 150 b are located to be generally coplanar withinsensor housing 120. In the illustrated embodiment, first workingelectrode 150 a is formed by depositing a first layer of catalyst 154 aon a diffusion membrane 152 (using, for example, catalyst depositiontechnique known in the sensor arts). Second working electrode 150 b isalso formed by depositing a second layer of catalyst 154 b on diffusionmembrane 152 (using, for example, catalyst deposition techniques knownin the sensor arts). Methods of fabricating electrodes on diffusionmembranes are, for example, described in U.S. Patent ApplicationPublication No. 2011/0100813. Catalyst layers 152 a and 152 b may or maynot be formed using the same electrocatalytic material. It is immaterialwhether second gas diffusion or working electrode 150 b is operated asan anode or cathode with respect to the operation of first gas diffusionor working electrode 150 a.

FIGS. 3A through 3D illustrate an embodiment of a sensor 210 that issimilar in design and operation to sensor 110. Like elements of sensor210 are numbered similarly to corresponding elements of sensor 110 withthe addition of 100 to the reference numbers of the elements of sensor210. As illustrated in FIG. 3A, reference electrode 270, counterelectrode 280 and electrolyte absorbent wicks 240 a, 240 b and 240 c aresupported within housing 220 via a support member 284. A printed circuitboard 292 is connected to housing 220 and may form a part of theelectronic circuitry of sensor 210.

As, for example, illustrated in FIGS. 3A and 3C, a housing lid 222includes a first gas inlet 230 a and a second gas inlet 230 b. First gasinlet 230 a and a second gas inlet 230 b may, for example, be in fluidconnection with an inlet system 204 (including, for example, one or moreinlets) formed in a housing 202 of an instrument or system 200 (see FIG.3B). First inlet 230 a can, for example, be designed for use inconnection with a first working electrode 250 a for an analyte gas suchas hydrogen sulfide. A first catalyst layer 254 a of first workingelectrode 250 a, which is deposited upon a first diffusion membrane 252a, may, for example, include iridium in the case that the analyte gas ishydrogen sulfide (H₂S). Second inlet 230 b is designed for use inconnection with the application of exhaled breath to second workingelectrode 250 b. Second working electrode 250 b is formed by depositionof a second catalyst layer 254 b upon a second diffusion membrane 252 b.Separate gas inlets 230 a and 230 b may, for example, be designed oroptimized for passage of two different gases. In that regard, first gasinlet 230 a may be optimized (for example, in dimension and/or shape)for the analyte gas of interest, while second gas inlet 230 b may beoptimized for a component of exhaled breath.

In the case of an aqueous electrolyte, the material(s) (which can be thesame or different) of the gas diffusion membranes can be generallyhydrophobic in nature to minimize or eliminate any flow of the aqueouselectrolyte therethrough. In the case of a non-aqueous (for example,organic) electrolyte, the material of the gas diffusion membranes can begenerally oleophobic in nature to minimize or eliminate any flow of thenon-aqueous electrolyte therethrough. The material(s) can also behydrophobic and oleophobic. Such materials are referred to as“multiphobic”. The materials can also be chemically or otherwise treatedto minimize or eliminate liquid electrolyte flow or leakagetherethrough.

In general, the term “hydrophobic” as used herein refers to materialsthat are substantially or completely resistant to wetting by water atpressures experienced within electrochemical sensors (and thus limitflow of aqueous electrolyte therethrough). In general, the term“oleophobic” as used herein refers to materials that are substantiallyor completely resistant to wetting by low-surface tension liquids suchas non-aqueous electrolyte systems at pressures experienced withinelectrochemical sensors (and thus limit flow of non-aqueous electrolytetherethrough). As used herein, the phrase “low-surface tension liquids”refers generally to liquids having a surface tension less than that ofwater. Hydrophobic, oleophobic, and multiphobic materials for use inelectrodes are, for example, discussed in U.S. Pat. No. 5,944,969.

Gas diffusion membranes for use herein can, for example, be formed frompolymeric materials such as, but not limited to, polytetrafluoroethylene(for example, GORETEX®), polyethylene or polyvinylidene fluoride (PVDF).Such polymeric materials can, for example, include a pore structuretherein that provides for gas diffusion therethrough.

In sensors 110 and 210, first working electrodes 150 a and 250 a share acommon electrolyte, a common counter electrode (180 and 280) and acommon reference electrode (170 and 270) with second working electrodes150 b and 250 b, respectively. In certain situations, depending, forexample, upon the analyte gas to be detected and the associatedelectrochemistry, it may not be desirable or possible to have a commonelectrolyte, counter electrode and/or reference electrode. FIG. 3Eillustrates another embodiment of a sensor 210′, which is similar inoperation and construction to sensors 110 and 210. Unlike sensors 110and 210, in the embodiment of 210′, first working electrode 150 a′ andsecond working electrode 150 b′ are positioned in separate cells withinhousing 2201 which are not in fluid connection. In this manner, adifferent electrolyte can be used in connection with electrolytesaturated wick materials 140 a′, 140 b′ and 140 c′ than the electrolyteused in connection with electrolyte saturated wick materials 140 a″, 140b″ and 140 c″. Likewise, reference electrode 170 a′ may be formeddifferently from reference electrode 170 b′, and/or counter electrode180 a′ may be formed differently from counter electrode 180 b′. In theillustrated embodiment, separate inlets 230 a′ and 230 b′ are formed ina common lid or cap 222′ to be in fluid connection with first workingelectrode 150 a′ and second working electrode 150 b′, respectively.

FIGS. 3F and 3G illustrate another embodiment of a sensor 310, which issimilar in operation and construction to sensors 110 and 210. Sensor 310includes a housing 320 having a gas inlet 330 (formed in a lid 322 ofsensor housing 320) for entry of analyte gas and human breath intosensor 110. In the illustrated embodiment, inlet 330 is formed as anextending slot in lid 322 and is in fluid connection with a gasdiffusion member 318. Gas diffusion member 318 is, for example, formedfrom a porous polymeric material and provides for relatively quicklateral diffusion of gas to a first working electrode 350 a (responsiveto the presence of analyte gas) and a second working electrode 350 b(responsive, for example, to the presence of human breath) to reduceresponse times of sensor 310. First working electrode 350 a, secondworking electrode 350 b, and remainder of the components of sensor 330,may, for example, be formed in the same manner as described above forworking electrode 150 a, second working electrode 150 b and theremainder of the components of sensor 110. Gas diffusion member 318 may,for example, be stiffer in construction than diffusion membrane 352 a offirst working electrode 350 a and diffusion membrane 352 b of secondworking electrode 350 b (upon which, catalyst layers 354 a and 354 b,respectively, are deposited). In addition to providing relatively quicklateral diffusion, gas diffusion member 318 may also protect diffusionmembranes 352 a and 352 b from “pinching” as a result of mechanicalcompression.

Although the transport paths for first working electrodes 250 a, 250 a′and 350 a and for second working electrodes 250 b, 250 b′ and 350 b ofsensor 210, 210′ and 310 are slightly different, all transport paths ina particular instrument experience generally the same environments andenvironmental conditions. Therefore, a challenge with a driving forcesuch as, for example, exhaled breath and the measured response of secondworking electrodes 250 b, 250 b′ and 350 b thereto provides anindication of the functionality of all transport paths in the system orinstrument.

In a number of embodiments described above, amperometric oxygen (orother) sensors operated in a diffusion mode are responsive to a drivingforce created in the vicinity of the inlet system (for example, exhaledbreath) to test one or more transport paths. Such sensors may also beused in an instrument with a plenum or manifold which supplies a testgas (via pumping) to one or more sensors or sensing elements in fluidconnection with the plenum. In this way, a single sensor responsive to adriving force such as exhaled breath provides information on the flowstate of all transport paths (including, for example, membranes andmembrane-protected or equipped sensors or sensing elements) in fluidcontact with the plenum. This is especially true if the sensorresponsive to the driving force such as exhaled breath is placedupstream of all the other sensors.

FIG. 3H illustrates an embodiment of an instrument or system 400including a plurality of individual sensors 410, 420, 430 and 440 withina common housing. At least one of sensors 410-440 may, for example, be anon-analytical oxygen sensor as described above which is responsive to,for example, oxygen concentration change resulting, for example, fromexhaled breath. In a number of embodiments, sensor 410, which is thefirst sensor in the flow path (that is, forced flow path), in system 400is, for example, a non-analytical oxygen sensor. In such an embodiment,sensors 420, 430 and 440 may, for example, independently be a sensor forthe detection of H₂S, CO₂, CO, NO₂, NO, SO₂, HCN, HCl, NH₃, H₂, CH₄,C₂H₄, Cl₂, EtOH or other analyte gases of interest. In a number ofembodiments, at least one of sensors 420, 430 and 440 is an analyticaloxygen sensor. Working electrodes 414, 424, 434, and 444, referenceelectrodes 416, 426, 436, and 446, counter electrodes 418, 428, 438, and448, as well as the remaining components of sensors 410, 420, 430, and440, respectively, may, for example, be formed in the manner describedabove. As is clear to one skilled in the art, system 400 may, forexample, include fewer than or greater than four sensors.

As used herein, “analytical”, “analytical electrode” and like termsrefer to a working or sensing electrode with sufficient characteristicsto provide an accurate or analytical indication of the concentration ofthe gas being sensed. Such characteristics include, for example,sufficient response range to provide accurate indications of test gascontent over the desired range of concentration, long-term baselinestability, resistance to changes resulting from changes in environmentalconditions, etc. “Non-analytical”, “pseudo-analytical” and like termsrefer to a working or sensing electrode with sufficient range andaccuracy to be useful to accomplish an exhaled breath test or other flowpath test as described herein. Stability and accuracy are not asimportant in this aspect as the exhaled breath test or other flow pathtest hereof occurs over a short time frame, and the response is entirelycontained within that time frame. That is, there is no need to refer toan earlier established calibration event.

Referring again to FIG. 3H, each of sensor 410, 420, 430 and 440 is influid connection with a plenum 402. Test gas from the ambientenvironment is forced through plenum 402 (in the direction of the arrowsof FIG. 3H—that is, entering plenum 402 via an inlet 402 a and exitingplenum 402 via an exit 402 b) via pump 406 including a pump motor 406 a.Pump 406 is in fluid connection with the ambient atmosphere and withplenum 402. Sensors 410, 420, 430 and 440 as well as pump 406 may, forexample, be in communicative connection with a control system which may,for example, include a processor system 404 (including, for example, oneor more microprocessors) and/or circuitry for control thereof and datacollection/processing. Processor system 404 is, for example, incommunicative connection with a memory system 405. System 400 furtherincludes at least one power source 408 (for example, one or morebatteries). System 400 may also include at least one user interfacesystem 409 in communicative connection with processor system 404 andmemory system 405 to provide information to a user. User interfacesystem 409 may, for example, include a display for visual signals.Information may also be provided via user interface system 409 viaaudible, tactile and/or olfactory signals.

As described above, in a number of embodiments, sensor 410 is anon-analytical oxygen sensor and one of sensors 420, 430 and 440 may bean analytical oxygen sensor. The output of the analytical oxygen sensorin ambient air (20.8 vol-% oxygen) provides an independent check of thehealth or state of function of system 400. Such an analytical oxygensensor may, for example, be used in any embodiment of systems hereof.

As illustrated in FIG. 3I, system 400 may also be operated in adiffusion mode when pump 406 a is not powered. In other embodiments, asensor housing with multiple separate sensors in fluid connection with acommon gas inlet may be provided in which no pump is present. Onceagain, separate and distinct electrochemical cells within a commonhousing including, for example, at least one sensor responsive to oxygenprovides a flow check or transport path functionality check as describedherein, wherein individual sensors may be formed without the designrestriction of common components (as, for example, illustrated inconnection with FIG. 2A). As described above, sensor 410 may includeoxygen sensitive chemistry (and components) described herein and othersensors 420, 430, 440 etc. may include entirely different sensingchemistry (and components) such as those described in U.S. Pat. Nos.5,944,969, 5,667,653, and elsewhere.

FIG. 3J illustrates another embodiment of instrument or system 400(which may operate in a forced flow or pumped mode and/or in a diffusionmode). In the embodiment of FIG. 3J, system 400 includes one or moreelectrochemical sensors 410, 420 and 430 and one or more combustible gassensors represented by combustible gas sensor 440 c. Catalytic orcombustible (flammable) gas sensors have been in use for many years to,for example, prevent accidents caused by the explosion of combustible orflammable gases. In general, combustible gas sensors operate bycatalytic oxidation of combustible gases. As illustrated in FIG. 3J,combustible gas sensor 440 c includes a sensing element 442 c, whichincludes a heating element such as a platinum heating element wire orcoil 442 c(i) encased in a refractory (for example, alumina) bead 442c(ii). Bead 442 c(ii) is impregnated with a catalyst (for example,palladium or platinum) to form active or sensing element 442 c, which issometimes referred to as a pelement, pellistor, or detector. A detaileddiscussion of pelements and catalytic combustible gas sensors whichinclude such pelements is found, for example, in Mosely, P. T. andTofield, B. C., ed., Solid State Gas Sensors, Adams Hilger Press,Bristol, England (1987). Combustible gas sensors are also discussedgenerally in Firth, J. G. et al., Combustion and Flame 21, 303 (1973)and in Cullis, C. F., and Firth, J. G., Eds., Detection and Measurementof Hazardous Gases, Heinemann, Exeter, 29 (1981).

Sensing element 442 c may react to phenomena other than catalyticoxidation that can change its output (i.e., anything that changes theenergy balance on the bead) and thereby create errors in the measurementof combustible gas concentration. Among these phenomena are changes inflow, ambient temperature, humidity, and pressure. To minimize theimpact of secondary effects on sensor output, the rate of oxidation ofthe combustible gas may be measured in terms of the variation inresistance of sensing element or pelement 442 c relative to a referenceresistance embodied in an inactive, compensating element or pelement 444c. The two resistances are typically part of a measurement circuit suchas a Wheatstone bridge. The output or the voltage developed across thebridge circuit when a combustible gas is present provides a measure ofthe concentration of the combustible gas. The characteristics ofcompensating pelement 444 c are typically matched as closely as possiblewith active or sensing pelement 442 c. Compensating pelement 444 c,however, typically either carries no catalyst or carries aninactivated/poisoned catalyst.

Active or sensing pelement 442 c and compensating pelement 446 c can,for example, be deployed within wells 446 c(i) and 446 c(ii) of anexplosion-proof housing section 448 c and can be separated from thesurrounding environment by a flashback arrestor, for example, a porousmetal frit 449 c. Porous metal frit 449 c allows ambient gases to passinto housing section 448 c but prevents ignition of flammable gas in thesurrounding environment by the hot elements. Such catalytic gas sensorsmay be mounted in instruments such as instrument 400 which, in somecases, must be portable and, therefore, carry their own power supply408. It may, therefore, be desirable to minimize the power consumptionof a catalytic gas sensor.

Combustible gas sensor 440 c may provide an additional (or analternative) sensor which is responsive to a flow path test as describedherein. As described above, combustible gas sensors are sensitive tochanges in flow, ambient temperature, humidity, and pressure. Moreover,combustible gas sensors are also sensitive to the concentration ofoxygen in the environment surrounding the sensing element. Multiplesensors (of the same or different types) which are responsive to one ormore driving forces of a flow path test hereof may, for example, bepositioned at various positions along one or more flow paths of a systemhereof to provide improved data specificity during a flow path test.

In several studies of sensors hereof, sensors fabricated in the mannerof sensor 210 hereof were studied wherein first gas diffusion or workingelectrode 250 a was used to detect hydrogen sulfide (H₂S), while secondgas diffusion or working electrode 250 b was used to detect the oxygencomponent of exhaled breath. Sensors fabricated in the manner of, forexample, sensor 110, sensor 210′, sensor 310 or sensor 410 would operatein the same or similar manner. In the specifically studied embodiments,first electrocatalyst layer 254 a included iridium (Ir) metal. Secondelectrocatalyst layer 254 b included platinum (Pt) metal, Otherelectrocatalysts suitable for reduction of oxygen may be used in secondelectrocatalyst layer 254 b.

FIG. 4 illustrates the behavior sensor 210 when challenged with exhaledbreath, followed by a mixture of 15 vol-% oxygen and 20 ppm hydrogensulfide, followed by nitrogen. The H₂S channel trace is the response offirst working electrode 250 a (designed to detect hydrogen sulfide), andthe O₂ channel trace is the response of second working electrode 250 b(designed to detect the oxygen component of exhaled breath). Asillustrated, second working electrode 250 b responds to the decreasedoxygen content of exhaled breath which occurs at approximately the 50second mark in the graph. A mixture of 15 vol-% oxygen and 20 ppmhydrogen sulfide was applied at approximately 100 seconds. Each of firstworking electrode 250 a and second working electrode 250 b respondedappropriately to this challenge gas. Finally, nitrogen was applied at250 seconds. Upon application of nitrogen, second working electrode 250b (designed for the detection of oxygen) responded appropriately to thechallenge gas.

The response of second working electrode 250 b to exhaled breath asshown in FIG. 4 may, for example, be used to determine that thetransport paths (including gas diffusion members and/or membranes) of aportable gas detection instrument are, for example, not compromised bydust, vapors, and/or liquid. That is, based on the response of secondworking electrode 250 b to the decreased oxygen concentration of exhaledbreath, it can be determined that there is appropriate flow through allgas diffusion members (for example, gas diffusion membranes 252 a and252 b), whether they are part of sensor 210 itself or part of theoverall instrument. This gas response, when combined with, for example,an internal sensor electronic interrogation signal (such as thatdescribed in U.S. Pat. No. 7,413,645), may be used to provide a check ofboth the internal conductive condition of an amperometricelectrochemical sensor (or other sensor) and any gas transport path(s)(including, for example, associated gas diffusion membranes), whetherpart of the sensor cell itself or part of the overall instrument. Inthis manner, a test similar in overall result to a bump test isaccomplished without the use of expensive and potentially hazardouscalibration gas and equipment associated therewith.

In a number of embodiments hereof for use in connection with an exhaledbreath test or bump check, an amperometric oxygen (or other gas) sensingelement is disposed within, for example, an amperometric toxic (orother) gas sensor for detecting an analyte of interest. In a number ofthe embodiments described above, both an analyte gas sensing workingelectrode and the oxygen sensing electrode are conventionally fabricatedas gas diffusion electrodes. In many cases, such gas diffusionelectrodes include a high surface area electrocatalyst dispersed on aporous support membrane. In embodiments in which an amperometric gassensor is used in systems hereof as a secondary sensor to test one ormore transport paths, because the secondary sensor (for example, anoxygen sensor) is not used to present an analytical signal (that is, itmay be a non-analytical sensor), there may be no need to use either agas diffusion electrode or a high surface area electrocatalyst.

For example, a conductor such as a contact ribbon or another conductivemember, which are often used to carry an electrical signal from a gasdiffusion electrode, may have sufficient surface area andelectrocatalytic activity to be used as an oxygen, CO₂ or other gassensitive electrode. For example, FIG. 5A illustrates a ribbon 450 a anda wire 450 a′ which may be used to form a non-analytical sensor elementin the systems hereof. Such ribbons or wires may, for example, befabricated from an electrocatalytic material such as Platinum (Pt),Iridium (Ir), Gold (Au) or carbon (C). As illustrated in FIG. 5B sensorelements 550 a and 550 a′ hereof may, for example, be a conductiveribbon 552 a or a conductive wire 552 a′, respectively, upon which anelectrocatalytic material 554 a and 554 a′ (for example, Pt, Ir, Au, Cetc.), respectively, is coated or immobilized. The material of ribbon552 a and wire 552 a′ may be the same or different from electrocatalyticmaterial 554 a and 554 a′ immobilized thereon.

The sensor elements or electrodes hereof for testing transport paths maytake a wide variety of two-dimensional or three-dimensional shapes. Forexample, FIG. 5C illustrates a sensor element 650 a hereof including anextending ribbon 652 a having a rectangular end member 653 a which iswider than extending ribbon 652 a to, for example, provide increasedsurface area per unit length as compared to a ribbon of the same length.Similarly, FIG. 5D illustrates a sensor element 650 a′ hereof includingan extending ribbon 652 a′ having a rectangular end member 653 a′. Inthe embodiment of FIG. 5D, an electrocatalytic material 654 a′ isimmobilized on end member 653 a′. FIG. 5E illustrates a sensor element750 a hereof including an extending wire 752 a having a spiraled section653 a on an end thereof, which may, for example, provide increasedsurface area per unit length as compared to an extending wire of thesame length. Similarly, FIG. 5F illustrates a sensor element 750 a′hereof including an extending wire 752 a′ having a spiraled section 753a on an end thereof. In the embodiment of FIG. 5F, an electrocatalyticmaterial 754 a′ is immobilized on spiraled section 753 a′. In theembodiments of FIGS. 5D and 5F, electrocatalytic materials 654 a′ and754 a′ may be the same or different as the material upon which theelectrocatalytic material is immobilized.

In the embodiments discussed above, a first electrode is used forsensing an analyte and a second electrode, formed separately from thefirst electrode, is used to, for example, detect oxygen concentration.In the representative example of a toxic gas sensor for detecting theanalyte H₂S, for example, the toxic gas channel (H₂S, in that case) isfabricated to include the electrocatalyst iridium (Ir) and theoxygen-sensing electrode is fabricated to include the electrocatalystplatinum (Pt). Those electrocatalysts may, for example, be independentlydispersed onto the same porous substrate, but in two distinct anddifferent areas. The same or similar functionality may, for example, beachieved if mixtures of Pt and Ir are used. For example, such mixturesmay be physical mixtures of high surface area catalytic powders or suchmixtures may be alloys. In a number of embodiments, one electrocatalyticsubstance or material may, for example, be fabricated on top of anotherelectrocatalytic substance or material in a two-step process.

Moreover, the two electrocatalytic materials may, for example, befabricated into an interdigitated electrode system. FIG. 6 illustratesan embodiment of an interdigitated electrode system 850 wherein a firstbranch 850 a of electrode system 850 includes a first electrocatalyticmaterial and a second branch 850 b includes a second electrocatalyticmaterial. The first and second electrocatalytic materials of the twobranches or “fingers” 850 a and 850 b of electrode system 850 may, forexample, be fabricated to include the same electrocatalytic substance(or mixture of substances) or to include different electrocatalyticsubstances.

In another embodiment of an electrode system 950 hereof illustrated inFIG. 7, a first electrode 950 a and a second electrode 950 b aresupported upon a gas porous disk 960, which is formed as an annulus inthe illustrated embodiment. Disk 960 may, for example, be fabricatedfrom gas porous or permeable (that is, adapted to transport gastherethrough) polymer or another material that is inert in theelectrolyte used in the sensor system. As described above, disk 960serves as an electrode support onto which first working electrode 950 aand secondary working electrodes 950 b are fabricated, but on oppositesides of disk 960 as illustrated in FIG. 7. First or upper electrode 950a (in the orientation of FIG. 7) is formed as an annulus. Second orbottom electrode 950 b is formed as a disk centered on the annulus ofdisk 960. Electrode system 950 further includes a first or upperelectrolyte wick 970 a and a second or lower electrolyte wick 970 b.Electrode system also includes a first electrode current collector 980 aand a second electrode current collector 980 b.

The configuration of FIG. 7 may, for example, be vertically flipped orrotated 180° from its illustrated orientation and still function asintended. Many other shapes and configuration of electrodes are possiblefor use herein. Moreover, electrodes hereof may, for example, be stackedin multiple layers or other arrangements to produce sensors with asensitivity for a multiplicity of target gases.

In a number of embodiments hereof, a single working or sensingelectrode, operated at a single bias potential, can be used thatresponds to both the analytical gas of interest (analyte) and to aanother driving force (for example, a component of exhaled breath) toenable testing of one or more transport paths to the electrode(s) of thesystem. For example, in the representative sensor system described inFIG. 2, the H₂S working electrode also responds to exhaled breath. Theresponse of the working electrode to exhaled breath can be used to testthe function of the transport path. FIG. 8A illustrates the response ofa representative embodiment of a single channel amperometric sensorhaving a single electrode, operated at a single bias potential,fabricated to include an electrocatalytic material that is responsive toan analytical gas of interest or analyte (H₂S in the representativeexample), to exhaled breath and to nitrogen. The electrode may befabricated from a single electrocatalytic material, a physical mixtureof electrocatalytic materials or an alloy of electrocatalytic materials.The data shown in FIG. 8A was collected by operating a hydrogen sulfide(H₂S) sensor at a constant bias potential of zero (0) mV versus aninternal reference electrode. At this potential the working electrode(iridium (Ir), in this case) is sufficiently anodic to cause theFaradaic conversion of hydrogen sulfide to sulfur dioxide (SO₂), as iswidely reported in the electrochemical literature. This can be seen inthe graph of FIG. 8A, beginning around the 100 second mark, andrepresents the analytical signal of the sensor. Prior to the applicationof hydrogen sulfide, a driving force was applied to the sensor in theform of exhaled breath. The associated sensor response can be seen atabout the 50 second mark in the graph. There is a small, positiveexcursion of the trace, upon application of exhaled breath, which wasprobably a result of the changes in the local humidity of the atmospherein fluid contact with the sensor, caused by the high humidity (near 98%RH) in exhaled breath. Finally, a second driving force was applied tothe sensor by the application of nitrogen (N₂) to the sensor at about250 seconds. Again there is an excursion in the sensor signal, both forthe application and removal of N₂. In this case, the signal originatesfrom non-Faradaic rearrangement of ions near the electrode surface as aresult of the sudden change in oxygen concentration. In both cases theapplication of a driving force to the face of the sensor, either byexhaled breath or by nitrogen, causes a sufficient, transitory change inthe sensor signal to be used to assess the condition of the flowelements and flow path into the sensor. As described above, theseeffects are observed on a single sensor, with one working electrode,operated at a single, constant bias potential.

FIG. 8B illustrates a portion of another embodiment of sensor 410 a thatis similar in design and operation to sensor 110. The remaining portionof sensor 410 a may, for example, be substantially identical in designto sensor 110. Like elements of sensor 410 a are numbered similarly tocorresponding elements of sensor 110 with the numerical addition of 300and the addition of the designation “a” to the reference numbers ofcorresponding elements of sensor 110. As illustrated in FIG. 8B, ahousing lid 422 a includes a gas inlet 430 a which may, for example, bedesigned for use in connection with a working electrode 450 a for ananalyte gas such as hydrogen sulfide. A catalyst layer 454 a of workingelectrode 450 a, which is deposited upon a first diffusion membrane 452a, may, for example, include iridium in the case that the analyte gas ishydrogen sulfide (H₂S).

In a number of the embodiments discussed above, one channel, forexample, a toxic gas channel for the measurement of H₂S is fabricated tohave a working electrode including an iridium catalyst, while a secondchannel includes an oxygen sensing electrode including a platinumcatalyst. As described above, those catalysts may, for example, beindependently dispersed on the same porous substrate in two distinctareas. In the embodiment of FIG. 8B working electrode 450 a is operatedat two bias potentials. At a first bias potential, working electrode 450a is active for oxidizing or reducing a target gas or analyte thatsensor 410 a is intended to detect (for example, H₂S). At a second biaspotential, which is different from the first bias potential, workingelectrode 450 a is active for oxidizing or reducing a component ofexhaled breath utilized in an exhaled breath check as described above.The bias switching described above is controlled by the drivingcircuitry (for example, included upon a printed circuit board such asprinted circuit board 292) and logic of sensor 410 a and/or aninstrument in which sensor 410 a is included. Gas inlet 430 a may, forexample, be optimized (for example, in dimension and/or shape) for theanalyte gas of interest and for a component of exhaled breath.

One of the more important operational aspects of using bias switching ina sensor with interrogation features as described above is that of thephenomenon colloquially known as “cookdown” to those skilled in the artof amperometric electrochemical gas sensors. Cookdown refers to thedecay of large extraneous (that is, extraneous to the application of gassensing) currents that flow between the working and counter electrodesof an amperometric gas sensor when the bias applied to the workingelectrode is suddenly changed (with respect to the either an internalreference electrode, in a three electrode cell, or with respect to acombination counter/reference electrode in a two electrode cell).

In the electrochemical arts, “Faradic current” usually refers tocurrents that flow in an electrochemical device when one substance iselectrochemically converted to another, such as, for example, in anoxidation-reduction reaction, such as the reduction of oxygen (O₂) towater in an acidic electrolyte:O₂+4H⁺+4e ⁻

2H₂O  1.1

Conversely, non-Faradaic currents are those currents that flow in anelectrochemical cell when no substance is converted and are a result ofonly the rearrangement of ions very close to the electrode surface.

These phenomena may become important in considering the behavior of asensor such as sensor 410 a that uses single working electrode 450 a,operated at two different bias potentials, to access the electrochemicalreaction important for sensing the gas of interest and to access thepotential region where, for example, oxygen (a component of exhaledbreath) is reduced according to equation 1.1, above, to enable anexhaled breath test or flow check. In the example of a sensor withinterrogation functionality described herein in which the intendedtarget gas to be sensed is hydrogen sulfide (H₂S), one would typicallyuse a high surface area iridium (Ir) electrocatalyst (Ir black) as theworking electrode surface. At an applied potential of zero (0) mV versusan iridium/air (Ir|air) or platinum/air (Pt|air) pseudo-referenceelectrode (as is commonly employed in sensors to sense H₂S including anIr working electrode), H₂S is oxidized to sulfur dioxide (SO₂) accordingto:H₂S+2H₂O

SO₂+6H⁺+6e ⁻  1.2

The above reaction is a Faradaic reaction, and occurs only where thereis H₂S in the atmosphere supplied to the sensor (for example, sensor 410a). In the absence of H₂S, very small (near zero) non-Faradaic currentsflow as a result of the continual rearrangement of ions very near theelectrode surfaces. Such ionic rearrangements are a result of thermallyinduced Brownian motion. The phenomenon of cookdown becomes importantwhen the potential or bias of the working electrode is suddenly changed.

As described above, the same high surface area Ir electrode (forexample, working electrode 450 in the embodiment of FIG. 8B) may also beused to sense oxygen in the atmosphere supplied to the sensor (accordingto equation 1.1) if the potential of the working electrode is changed toapproximately −600 mV (versus the internal reference electrode describedabove). This sudden change results in large negative currents (followingthe convention that reduction currents are presented as negative) thatdecay slowly over time to a steady state current that is indicative ofthe amount of oxygen present in the atmosphere sensed by the device. Thedecay over time is referred to as “cookdown.” In the case describedabove, there are two sources of the cookdown current. The first sourceof current is the electrochemically induced rearrangement of ions verynear the electrode surface as a result of the newly applied potential,which is a non-Faradaic current. The second source of current is theelectrochemical reduction of oxygen. The oxygen that iselectrochemically reduced includes oxygen that is dissolved in theelectrolyte of the sensor. In that regard, until the step change inpotential to −600 mV, the electrode was operated in a region where theconversion depicted in equation 1.1 did not occur. Therefore, over time,the electrolyte becomes saturated with dissolved oxygen. Theelectrochemically reduced oxygen also includes the oxygen being suppliedto the working electrode from the atmosphere applied to the sensor. Theresultant current is Faradaic current, resulting from the conversion ofoxygen to water, regardless of the source of oxygen. The currentresulting from reduction of oxygen dissolved in the electrolyte may beaccounted for during an exhaled breath test.

Operated at a potential of −600 mV, a sensor with an interrogationfunctionality as described herein is able to undergo or perform sometype of breath or flow check that involves the perturbation of deliveryof oxygen to the sensor. This may, for example, be associated with theapplication of exhaled breath.

During operation, a sensor such as sensor 410 a would be operated at−600 mV only during an exhaled breath or flow test/check. Its nominaloperation would be at an applied potential of zero mV for the sensing ofH₂S. However, upon the completion of the exhaled breath or flowtest/check, the external operational circuitry of sensor 410 wouldsuddenly return the applied bias of working electrode 450 a to 0 mV.This bias potential change would induce large, transitory positivecurrents, until sensor 410 a returned to its normal, near zero currentin the absence of H₂S. The large, positive, transitory current would bethe non-Faradaic cookdown of sensor 410 to its normal operating state.Such cookdown currents will occur every time the bias is switched to andfrom the region where sensor 410 would reduce oxygen, as is necessaryfor the exhaled breath or flow test/check.

FIGS. 8C and 8D provide data from examples of electrochemical sensorssuch as electrochemical sensor 410 a in which a single working electrodeis operated at two different bias potentials. As described above, thesensor is operated at a first bias potential for the analytical sensingof the analyte gas of interest, and at a second, different biaspotential, for operation as a pseudo-electrode for oxygen. In thiscontext, the term “pseudo-electrode” refers to a working electrode-biaspotential combination that gives a sufficient response to indicate achange in the oxygen content of the atmosphere being sensed, but may nothave the analytical sensitivity or range to be relied upon to provide ananalytical or accurate indication of the oxygen content of theatmosphere. Thus, when operated as a pseudo-electrode, the workingelectrode may be operating as a non-analytical electrode as describedabove.

FIG. 8C provides an example of operation of a sensor for the detectionof the analyte hydrogen sulfide at two different biasing potentials. Asdescribed above, the tested sensor utilized an iridium (Ir) workingelectrode, operated a two different potentials to accomplish both theanalytical detection of hydrogen sulfide and to act as apseudo-electrode for detecting a change in the oxygen content of theatmosphere being detected (such as, for example, during an exhaledbreath check). In FIG. 8C, the upper, solid line represents the resultsof the application of 20 ppm H₂S to the sensor with the Ir workingelectrode biased at 0 mV verses the internal Pt|air reference electrode,plotted against the left-hand, y-axis. The lower, broken-line tracerepresents the response of the same electrode, operated at −600 mV(versus the same internal reference electrode), plotted against theright-hand, y-axis to five consecutive 2.5 second applications ofnitrogen. Referring to the broken line in FIG. 4B, operation of the Irworking electrode at −600 mV, in air (20.8 vol-% oxygen) resulted in acurrent, from the electrochemical reduction of oxygen in air, ofapproximately −27 μA. The 2.5 sec pulses of nitrogen (simulating oxygenreduction associates with, for example, an exhaled breath test asdescribed herein) resulted in positive deflections of the oxygenbaseline, sufficient in magnitude to act as an indication of thecondition of the flow system flow path elements of the sensor.

FIG. 8D provides data from an experiment similar to that of FIG. 8C, butfor a carbon monoxide sensor. In the sensor of the experiments of FIG.8D, the working electrode was platinum (Pt). The upper, solid line setsforth the results of the application of 60 ppm CO to the Pt workingelectrode, biased at 0 mV against the internal Pt|air referenceelectrode, plotted against the left-hand, y-axis. The lower, broken linesets forth the results of the same working electrode, operated at −600mV against the internal reference electrode, plotted against theright-hand, y-axis. In FIG. 8D, the current observed as a result of thereduction of oxygen from the atmosphere results in a steady state orbaseline oxygen reduction current of approximately −5300 μA. Thepositive pulses in FIG. 8D were the result of five consecutive 2.5second long pulses of nitrogen. The positive deflections observed inFIG. 4D were sufficient to assess the condition of the flow paths of thesensor.

Many other types of sensor may include a working electrode operated attwo potentials as described above. For example, similar behavior isobserved for a chlorine (Cl₂) or a chlorine dioxide (ClO₂) sensorutilizing a gold (Au) working electrode. Further, a sulfur dioxide (SO₂)sensor with either platinum or gold working electrodes could be operatedin the same manner.

In a number of other embodiments of sensor systems hereof, two sensingor working electrodes are provided which include the sameelectrocatalytic material immobilized thereon. The electrodes can, forexample, be fabricated in an identical manner. In such embodiments, theanalyte gas and, for example, a gas of interest in exhaled breath areeach electroactive on the electrocatalytic material. In a number ofembodiments, the function of the two electrodes is alternated (forexample, each time the user activates a breath check as describedabove). Referring to, for example, FIG. 6, the first and secondelectrocatalytic materials of the two branches or electrodes 850 a and850 b of electrode system 850 would include the same electrocatalyticmaterial. In a first instance of activation of the instrument includingelectrodes 850 a and 850 b, electrode 850 a would be used as the workingelectrode for the target analyte gas and electrode 850 b would be usedto, for example, detect a component of exhaled breath (for example,oxygen). The next time the user activates the internal breath check (asecond instance), the functions of electrodes 850 a and 850 b would beswitched by the external circuitry and logic of the system or instrumentincluding sensors 850 a and 850 b. That is, in the second instance,electrode 850 b would be used as the working electrode for the targetanalyte gas and electrode 850 a would be used to detect the component ofexhaled breath. In this manner, alternatively, each electrode area wouldbe calibrated against the target gas of interest and the electronic lifeand health checks described below would be periodically applied to eachelectrode. Such a system and methodology provides a greater amount ofsurveillance and surety to the test methodology. A detection or sensingelement switching scheme which may be adapted for user herein isdescribed in U.S. Patent Application Publication No. 2011/0100090, thedisclosure of which is incorporated herein by reference.

The application of human breath to cause a perturbation in, for example,oxygen concentration as described herein is applicable, in mostinstances, to portable instrument applications, wherein a human user isavailable to provide a sample of exhaled breath to exercise theinterrogation or test system of the sensor (as described above), therebytesting flow through the instrument and/or sensor inlet holes andmembranes (that is, testing flow paths of the system). Analysis ofoxygen concentration perturbation may also be extended to, for example,permanent sensing applications (in which a sensor is fixedly positionedfor extended periods of time—typically until replacement), wherein thereis no human user available to exhale breath into the instrument/sensormembranes. The instrument may, for example, be placed in a positionwhich is not easily accessible by a human attendant.

FIGS. 9A and 9B illustrate schematically an embodiment hereof in which agas such as oxygen in the ambient atmosphere is used to test orinterrogate a system via dynamic coulometric measurement. In theillustrated embodiment, a sensor such as sensor 110 described above isprovided. Along with sensor 110 and other components of a permanentsensing application (as known to those skilled in the art, and which aresimilar to those describe above for portable sensor systems), a smallvolume or space (sometimes referred to herein as a diffusion volume) andan associated restrictor system or mechanism (that is, a system ormechanism which restricts of or limits (including eliminating) flow ofmolecules into the volume) are situated immediately adjacent to sensor110. The diffusion volume may, for example, incorporate allflow/diffusion paths into the sensor, including dust covers, filters,etc. The diffusion volume is relatively small and in no way interfereswith the normal operation of sensor 110, including normal sensing andcalibration. However, the diffusion volume is provided with a restrictormechanism that, when applied, may, for example, create a small sealedvolume immediately adjacent to the inlet diffusion means of sensor 110,thereby disrupting flow/diffusion of oxygen from the ambient atmosphereinto the volume. In the embodiment of FIG. 9A, sensor housing 120 is atleast partially encompassed within a secondary housing or cap 120′ tocreate a volume 124′ adjacent inlet 130 formed in lid 122′ of sensorhousing 120.

FIG. 9A illustrates sensor 110 and diffusion volume 124′ in normal(fully open) operation wherein volume 124′ and sensor inlet 130 are influid connection with the ambient atmosphere in which the concentrationof an analyte is to be tested. FIG. 9B illustrates sensor 110 and volume124′ in a restricted, closed or interrogation/testing position via acontrollable restrictor mechanism, closure or lid 122′, which may becontrollably altered between a fully open state as illustrated in FIG.9A and a flow/diffusion restricted or closed state as illustrated inFIG. 9B. In permanent sensing applications, restrictor mechanism 122′may, for example, be actuated remotely, either by user input, orautomatically, by the sensing device itself via a local and/or remotecontrol system 150′ illustrated schematically in FIG. 9B. Like otherFigures hereof, FIGS. 9A and 9B are not necessarily drawn to scale.FIGS. 9C and 9D illustrate a cross-sectional view of sensor 110incorporated within instrument or system 100 wherein closure 122′ (seeFIG. 9D) is an open state and a flow/diffusion restricted or closedstate, respectively (FIGS. 9C and 9D are not necessarily drawn toscale). As illustrated in FIG. 9D, alternatively, a restrictor mechanism104 a (illustrated in dashed lines) may be provided to restrict or closeinlet 104 of system 100 so that the created diffusion volumeincorporates all diffusion paths into sensor 110, including dust covers,filters, etc.

With closure 112′ (or restrictor mechanism 104 a) in an open state, andthe absence of an alarm condition, with a nominal signal present on theoxygen sensitive channel of sensor 110, and with a nominal response tothe electronic sensor interrogation system described below, it is highlylikely that there is present in (diffusion) volume 124′ (adjacent tosensor 130), ambient air with an oxygen concentration of approximately20.8 vol-%. Upon actuation of restrictor mechanism 122′ (or restrictormechanism 104 a) to place it in, for example, a closed state asillustrated in FIG. 9D, there is created, immediately adjacent to sensorinlet 130, a small, closed and fixed volume 124′ with a trapped volumeof gas containing a fixed amount of oxygen. That amount of oxygen willbe consumed by the oxygen sensitive channel of sensor 110 (describedabove), resulting in an asymptotically decreasing signal on the oxygensensitive channel. In a number of embodiments hereof volume 124′ ismaintained relatively small to ensure a relatively quick depletion ofthe oxygen therein. For example, in a number of embodiment, volume 124′is in the range of 0.25 to 1.5 ml. In a number of embodiments, volume124′ is approximately 0.5 ml.

It is not necessary to completely close the diffusion volume 124′adjacent to sensor 110, but it is only necessary to sufficiently disruptor restrict the diffusion of oxygen to the oxygen sensitive channel ofsensor 110 to cause a change in signal that can be analyzed according tothe principals of analytical coulometry, as described below. Altering orcycling restrictor mechanism 112′ (or restrictor mechanism 104 a)between an open, a closed, or a restricted state provides differentialdata, all of which can be deconvoluted to assess the condition of theflow path and flow elements into sensor 110.

Coulometry, as described above, is an analytical electrochemicaltechnique fundamentally involving the measurement of the passage ofcharge, in coulombs, involving a Faradaic conversion of substance, thatis, electrolysis. The measurement of charge is a fundamental (as opposedto derived) measurement, and therefore, can be used to make absolutequantitative analytical measurements.

Coulometry, or coulometric measurement, is typically performed using acoulometer, either electronic or electrochemical. Typically, coulometryis performed at constant potential and is often referred to as “bulkelectrolysis.” Given a well behaved electrochemical reaction, presentedin the general form:Ox+ne⁻→Red  1.3

a system can be easily set to reduce the oxidized species (Ox) at aconstant potential until it is completely converted to the reducedspecies (Red). This is signaled by a drop in observed current to zero.The amount of electricity (the number of coulombs) necessary to causethis conversion is a direct measurement of the amount of oxidizedspecies originally present in the system.

There are a number of ways in which a system can be modulated ordynamically changed to perform a coulometric measurement in a shortertime than by completing bulk electrolysis. For the particular systemsdescribed herein, a volume of gas in communication with an oxygen sensoris suddenly closed off from the ambient atmosphere (wherein diffusion ofthe analytically important species or analyte is modulated). The oxygenin the trapped sample is electrochemically consumed by the sensor (viaworking electrode 150 b in the representative example) according to:O₂+4H⁺+4e ⁻

2H₂O  1.4

If the volume of the sample is known, the absolute concentration ofoxygen in the trapped sample can be calculated based on the chargenecessary to completely consume it. Other techniques might be used toestimate the oxygen concentration including, for example, the rate ofdecay of the reduction current, or time to reach a predeterminedfraction of the original, un-modulated current. Many other schemes mightbe imagined. The theory behind dynamic measurements is discussed, forexample, in Stetter, J. R. and Zaromb, S., J. Electroanal. Chem., 148,(1983), 271, the disclosure of which is incorporated herein byreference.

At least three system conditions for the systems described herein can berelated to the response of the oxygen sensitive channel of the sensor(for example, via a processing system 192 including appropriatecircuitry and/or one or more processors such as a microprocessor). Eachof those conditions and the corresponding oxygen channel response/outputis illustrated in FIG. 9E. For normal operation, including an initial20.8 vol-% oxygen (see discussion above) and nominally open membranes, asignal decay curve similar to that labeled “Normal Response” isobtained. In this situation, the velocity of the signal decay isdependent only on the speed with which the trapped amount of oxygendiffuses to the oxygen sensitive element of the sensor (for example,working electrode 150 b of sensor 110) and is consumed by theelectrochemical reaction there present.

In the situation wherein the diffusion membrane(s) of the sensor inletare blocked by dust, or other foreign matter, the rate of diffusion ofoxygen to the sensor is decreased and a signal response similar to thatlabeled “Membrane Blocked” is obtained.

Alternatively, in the case of permanent sensing applications, it ispossible that bulk matter may become deposited in the diffusion volume(for example, volume 124′), however small it may be. This may, forexample, occur when an insect nest or the like occludes the face of thesensor. This situation is depicted in the signal response labeled“Diffusion Space Blocked.” In this case, the gas volume trapped in thediffusion volume is reduced from the normal case by the bulk matterpresent in the closed diffusion space and the response is observed todrop more quickly than the normal response.

In the case that oxygen variation (for example, as a result of a breathtest or a flow/diffusion restriction test) is measured, sensing elementsother than amperometric oxygen sensing element may, for example, beused. In that regard, any alternative oxygen sensing system may be usedin place of an amperometric oxygen sensing. Representative examples ofsuitable oxygen sensing systems include, but are not limited to, a metaloxide semiconductor or MOS (also colloquially referred to as a “Figaro”sensor) oxygen sensing element, a high temperature potentiometric oxygensensor (zirconia sensor), a combustible gas sensor, or a paramagneticoxygen sensor. A particular oxygen sensing technology may, for example,be more suitable as a complement to a given toxic gas or other sensingtechnology for a particular use. For example, an MOS or zirconia-basedoxygen sensing element may be well suited for use with an MOS toxicsensor or with a heated catalytic bead combustible gas sensor.

FIG. 10 illustrates a decision tree diagram that depicts arepresentative embodiment of an operating mode or method for use inconnection with sensors for an analyte in any of the systems hereof. Themethod illustrated in FIG. 10 assumes a successful complete calibrationof the instrument (with a calibration gas) at some point in time, eitherat final assembly and testing or in the field. In daily use, when theinstrument is turned on, as is typical, the instrument will perform itsnecessary self-diagnosis checks. Part of this self-diagnosis may, forexample, include the application of an electronic interrogation of asensor such as, for example, a life and health check similar to thatdescribed in U.S. Pat. No. 7,413,645.

As described in U.S. Pat. No. 7,413,645, and as illustrated in FIG. 11,a sensor generally can be described as a combination of resistances andcapacitance in series. The resistance R_(R) resulting from the referenceelectrode of FIG. 11 is not part of the current path of the analyticalsignal of the sensor. The resistive portion of this circuit is primarilya result of the solution (ionic) resistance of the electrolyteinterspersed between the working electrode (R_(W)) and the counterelectrode (R_(C)). The capacitive portion (C_(W)) of the equivalentcircuit is primarily a result of the micro solution environment foundvery close to the surfaces of the metallic particles that comprise theworking electrode. As a result of electrostatic forces, the volume ofsolution very close to the electrode surface is a very highly orderedstructure. This structure is important to understanding electrodeprocesses. The volume of solution very close to the electrode surface isvariously referred to as the diffusion layer, diffuse layer, and or theHelmholtz layer or plane.

The magnitudes of the resistance and capacitance present in anelectrochemical cell are a result of the nature and identities of thematerials used in its fabrication. The resistance of the electrolyte isa result of the number and types of ions dissolved in the solvent. Thecapacitance of the electrode is primarily a function of the effectivesurface area of the electrocatalyst. In an ideal world, these quantitiesare invariant. However, the solution resistance present in anamperometric gas sensor that utilizes an aqueous (water-based)electrolyte may change, for example, as a result of exposure todifferent ambient relative humidity levels. As water transpires from thesensor, the chemical concentration of the ionic electrolyte increases.This concentration change can lead to increases or decreases in theresistivity of the electrolyte, depending on the actual electrolyteused.

The response curves of sensors have the shape expected for the chargingcurve of a capacitor, that is a typical “RC” curve. In a number ofembodiments, the analytical signal used to determine the “health” of asensor is the algebraic difference in the observed potential just priorto the application of a current pulse and at the end of the currentpulse. The magnitude of the potential difference observed as a functionof the application of the current pulse is an indicator of the presenceand the health of any sensor of the system hereof and provides anindependent check of sensor system operability.

Although limitations on the magnitude and duration of the current pulsehave mostly to do with experimental convenience, the magnitude of thecurrent pulse may, for example, be chosen to correspond to applicationof a reasonably expected amount of target gas.

Sensor presence and health may be determined by the shape of thesensor's RC charging curve, being measured by observing the differencein sensor output at the beginning and the end of the current pulse. Ifthe sensor is absent, the observed potential is equal to that whichwould be expected based on the magnitudes of the current pulse and thesensor load resistor.

FIG. 12 illustrates a block diagram of one embodiment of an electronicinterrogation circuit as described in U.S. Pat. No. 7,413,645 and asused in several embodiments of the systems described herein. In FIG. 12,the voltage follower and the current follower sections function as knownto one skilled in the art. See, for example, A. J. Bard and L. R.Faulkner, Electrochemical Methods: Fundamentals and Applications, JohnWiley & Sons: New York (1980), the disclosure of which is incorporatedherein by reference. The voltage follower maintains a constant potentialbetween the reference electrode (R) and the working electrode (W). Thecurrent follower buffers and amplifies currents which flow in theelectrochemical sensor between the counter electrode (C) and the workingelectrode (W). In an number of embodiments, the current pump applieselectronic interrogation to the sensor by forcing a known current toflow between the counter electrode (C) and the working electrode (W).

On or more additional electronic interrogation tests may, for example,be performed on one or more combustible gas sensors in an instrument.For example, U.S. patent application Ser. No. 13/795,452, filed Mar. 12,2013, and entitled DIAGNOSTICS FOR CATALYTIC STRUCTURES AND COMBUSTIBLEGAS SENSORS INCLUDING CATALYTIC STRUCTURES, the disclosure of which isincorporated herein by reference, discloses an electronic interrogationtest for a sensing element of a combustible gas sensor in which avariable related to reactance of the sensing element is measured, andthe measured variable is related to an operational state orfunctionality of the sensing element.

In a number of embodiments hereof wherein an electronic interrogation asdescribed above or another electronic interrogation is used, redundantanalytical sensors (that is, redundant analytical sensors for the sameanalyte) may facilitate continuous sensing of the analyte. For example,a two channel amperometric electrochemical sensor with redundant,identical analytical channels may be used. The electronic interrogationmay, for example, be applied independently to each channel, in turn. Inthis embodiment, the benefits of electronic interrogation are obtained.However, because of the redundant, identical analytical channels, at notime would the sensing capability of the sensor for the analyte sensedby the redundant sensor be affected. Such embodiments might beparticularly useful for permanent sensor system installations, or forany sensor installation wherein the analytical signal of the sensorsystem for a particular analyte cannot be interrupted, even for theshort times necessary for the electronic interrogation described hereinor another electronic interrogation.

In a representative embodiment, a redundant carbon monoxide sensorsystem may, for example, include two independent platinum (Pt) workingelectrodes, a first working electrode and second working electrode,which may, for example, be dispersed on the same porous electrodesupport. Each working electrode is operated independently of the other,providing redundant indication of the absence or the presence andconcentration of carbon monoxide applied to the sensor. At somepredetermined time, either manually, remotely, or automatically, thefirst working electrode would undergo the electronic interrogationcheck, and the information necessary for the real-time correction of theanalytical signal and/or maintenance of channel 1 would be collected.The second working electrode/channel 2 would be completely unaffected bythis operation on channel 1. Sometime after the completion of theelectronic interrogation of the first working electrode/channel 1, afterthe effects of the interrogation have passed and a correct baseline isre-established, the second working electrode/channel 2 would undergo thesame electronic interrogation and signal collection, and the same datawould be obtained for channel 2. In this way, at no time is theanalytical signal for carbon monoxide from the sensor interrupted. Thisredundant working electrode/channel configuration may, for example, beutilized in connection with electronic or other interrogationsprocedures other than the electronic interrogation described inconnection with FIGS. 10 through 12. Moreover, the configuration isapplicable to sensors other than electrochemical sensors (for example,combustible gas sensors).

A further embodiment is illustrated in FIGS. 13A and 13B. In thisembodiment, a sensor 410 e includes a first working electrode 450 e(i)and second working electrode 450 e(ii), which are identical analyticalelectrodes as described above. Sensor 410 e further includes a thirdworking electrode 450 e(iii), which is a pseudo-analytical ornon-analytical electrode that is responsive to a driving force appliedin the vicinity of an inlet 430 e formed in an upper portion 422 e ofthe sensor housing to effect a flow path test hereof. As describedabove, third working electrode 450 e(iii) may, for example, beresponsive to some component of exhaled breath (oxygen, for example). Ina number of embodiments, all three working electrodes 450 e(i), 450e(ii) and 450 e(iii) are dispersed on the same porous support ordiffusion membrane 452 e. Working electrodes 450 e(i), 450 e(ii) and 450e(iii) are operated independently of each other, however. Workingelectrodes 450 e(i), 450 e(ii) and 450 e(iii) are in simultaneous fluidconnection with the atmosphere being sensed via V-shaped gas inlet 430e. Alternatively, each channel may have a separate gas inlet, or thevarious channels may be in fluid contact with the atmosphere beingsensed in a multitude of combinations. Working electrode 450 e(iii) may,for example, be used the perform a flow path test as described hereinby, for example, applying a driving force to inlet 430 e of sensor 410 e(for example, by applying exhaled breath). Working electrode 450 e(iii)would respond as previously described and would provide an indication ofthe operative state or functionality of the flow path into sensor 410 e.

Following an electronic interrogation test as described above as anindependent check of sensor health, the user may, for example, beprompted to perform a flow path test such as an exhaled breath test or a“bump check” hereof (without calibration gas) by exhaling closely intothe instrument face. Embedded instrument software observes the resultingsignal on, for example, second working electrode 250 b (designed torespond to some driving force/variable change associated with exhaledbreath such as a change in oxygen concentration). In the embodiment ofsensor 210, the observed response is a result of the decreased oxygencontent in exhaled human breath. The embedded instrument controlsoftware compares the result of the electronic interrogation test andthe result of the exhaled breath test to established parameters. If theresponses of either the electronic interrogation test or the flowpath/exhaled breath test fail to meet these pre-established criteria,the instrument may prompt the user to perform a full calibration or someother maintenance. If the results of both the electronic interrogationtest and the flow path/exhaled breath test meet or exceed thepre-established criteria, the instrument may indicate to the user thatit is functioning properly and is ready for daily use.

FIGS. 14A through 14C illustrate further embodiments of operatingmethodologies or schemes for systems hereof FIGS. 14A and 14B illustrateoperating methodologies in which both a life and health test (that is, atest including an electronic interrogation or stimulation of a sensor)and a flow path test or interrogation hereof are performed. In theoperating methodology of FIG. 14A, the electronic interrogation orsensor life and health test is performed first and the flow path test isperformed thereafter. In the operating methodology of FIG. 14B, the flowpath test or interrogation is preformed first and the electronicinterrogation or sensor life and health test is performed thereafter. Inthe operating methodology of FIG. 14C, only a life and health orelectronic interrogation test is performed.

In an initial, set-up phase, a user may be provided with the ability toadjust certain limits and set points prior to using either the sensorelectronic interrogation feature or the flow path test feature. Examplesof such adjustments include, but are not limited to, changes betweencalculated and calibrated sensitivity and time since last calibration.Once the initial set-up is complete, the user may, for example, beginusing the sensor interrogation features. A user may, for example, beginby initiating one of the interrogation methods. Alternatively,initiation of one of the interrogation methods may be set up toautomatically occur after a certain length of time, at a certain date,at a certain time of day, etc. User initiation may, for example, becarried out in many different manners including, for example, actuatinga button, transmitting a wireless command etc. Upon initiation, thesystem or instrument begins the test or interrogation process. Theinstrument then analyzes the data collected during the test process. Thesystem or instrument (via, for example, a control system which mayinclude a processor and/or other control circuitry) applies apredetermined algorithm or formula to the data and then compares theresults from the algorithm or formula to the set points or thresholdsearlier established (for example, during set up).

If the data from, for example, an electronic interrogation or life andhealth test of a sensor is “non-conforming” or outside of one or moredetermined set points or thresholds, one or more of the followingrepresentative tasks may be performed either individually or in anycombination: a) perform an automatic or automated (that is, without userintervention) gas calibration of the sensor, b) change the reportingparameters of the sensor/instrument, c) switch to a second sensingelement in the sensor or a new sensor for a particular analyte, d)signal the user to perform a “gas calibration” or perform othermaintenance, e) perform automated maintenance internal or external tothe sensor system, and/or f) signal to the user an “end of life” errormessage. For options “a”, “b”, “f” the user may, for example, beprovided a code providing information of what changes have occurred oranother indication of any changes. Such information may be simplycommunicated to the user or the system may require a user'sacknowledgement or approval of the changes. For options “c” and “d”, thesystem may, for example, require the user or instrument to repeat the“interrogation method” or signal the user or system to perform a “gascalibration”.

In the case of an automatic or automated gas calibration, the user neednot supply the gas or otherwise intervene. In that regard, a compressedgas container may be present in the vicinity of a permanent instrument.Alternatively, the test or calibration gas may be a generated in situ orotherwise released in a manner to enter the inlet of the instrument. Insitu gas generation is well know to those skilled in the art. Forexample, hydrogen gas (H₂) can easily be generated from anelectrochemical gas generator, which then can be used to calibrate bothhydrogen and carbon monoxide electrochemical sensors. Other gases ofinterest such as chlorine (Cl₂) and chlorine dioxide (ClO₂) can beelectro-generated as well. Also, there are methods of storing a gas ofinterest in a solid matrix from which it can then be thermally released.After such an automated calibration, the user may be provided with anindication of any system parameter changes, error codes and/or thereadiness of the instrument for further use.

In the case of changing the parameters of the sensor/instrument,parameters such as gain, range or resolution, cross-sensitivityparameters, set points (for example, alarm set point), alarm signals(for example, the type of signal) and/or other parameters may beadjusted for one or more sensors of an instrument on the basis of theresults of an interrogation method hereof. For example, based on thoseresults, the resolution of, for example, an H₂S sensor or other sensorof the instrument may be changed from 0.01 ppm to 0.1 ppm. Otherparameters that can be changed based on the results of interrogationmethods would include, but would not be limited to, changing the linearrange of the sensor so that gas values above or below certain levelwould not be displayed or reported or be displayed or reported on adifferent format. Additionally, any corrections to the linearity of thesensor signal that are normally applied may be altered or adjusted basedon the results of interrogation events. The electronic gain oramplification applied to the signal of a sensor may also be adjusted inthe same manner. A set point for an alarm threshold may be changed.Likewise, the alarm type to be provided to a user may also oralternatively be changed. The user may be informed of such a change asdescribed above. Such a parameter change or other parameter change may,for example, be made until the next interrogation or until the next gascalibration. Once again, the user may be informed of the change and maybe requested to acknowledge the change.

In the case of a multi-sensor system such as system 400 of FIGS. 3H and3I, redundancy of sensors may be provided. In that regard, ifnon-conforming results are obtained in an electronic interrogation of asensor, the system may switch to a second sensing element in the sensor.Alternatively, a user may be alerted to remove the non-conforminganalytical sensor and replace that sensor with a new sensor for aparticular analyte. After switching to the second sensing element, thesystem may repeat the interrogation and/or a gas calibration may beperformed (either an automated calibration or a user assistedcalibration).

Moreover, if a sensor is determined to no longer be suitable fordetection of a particular analyte, it may be suitable for detection ofanother, different analyte at the same or at a different biasingpotential. The system may, for example, switch the sensor to detectionof a different analyte in an automated procedure. The biasing potentialof the sensor may, for example, be changed to facilitate the sensing ofthe different analyte. As with other changes, the user may, for example,be notified and may be required to acknowledge or approve the change.

In the case that the instrument signals the user to perform a gascalibration, the user will supply a test gas (for example, a gasincluding a known concentration of the analyte or a simulant therefor)to the instrument inlet. User initiated maintenance might include, forexample, changing filters or dust covers. Many electrochemical sensorsare equipped with external chemical filters to remove interfering gases(see, for example, FIG. 1). The data from an electronic interrogation ofa sensor and/or a flow path test may, for example, be used to signal theuser that such a filter needs to be replaced. In the same way, manyinstruments, especially portable instruments, come equipped with filtersor dust covers that protect the sensor and the internals of theinstrument from intrusion of water, dust and other foreign materials.These dust covers and filters normally include at least part of the flowpath into the sensor. The data from an electronic interrogation of asensor and/or a flow path test may, for example, be used to signal theuser that such filters or dust covers need to be replaced.

Upon a certain result or combination of results from electronicinterrogation, the instrument or system may initiate an automatedmaintenance procedure. For example, the bias potential of anelectrochemical sensor may be altered via the instrument/system controlsystem or controller. The bias potential of the working or sensingelectrode of the electrochemical sensor may, for example, be altered 1)to increase its sensitivity to the target analyte, 2) to enhance theworking electrode's ability to discriminate against an interfering gas(that is, a gas to which the working electrode is responsive other thanthe analyte of interest). Moreover, a regeneration procedure may beinitiated. The biasing potential of the working or sensing electrodemay, for example, be changed to remove (for example, via oxidation,reduction, or desorption), an interfering or inhibiting substance thatmay have formed on or near (or otherwise contaminated) the sensingelectrode surface as a function of normal usage or as a result toexposure to an inhibiting agent or poison For example, the biasingpotential of a sensing electrode may be changed for a period of time andthen brought back to a potential at which the sensing electrode issensitive to the target analyte. For example, a CO sensor which istypically operated at a bias potential of zero (0) mV may have itsbiasing potential increase to +500 mV for a period of time (for example,one hour). Subsequently, the sensing electrode is returned to itsoperating biasing potential of zero (0) mV. This procedure may, forexample, improve cross-sensitivity to hydrogen (H₂). In the case of acombustible gas sensor, the temperature of the sensing element may beincreased for a period of time to “burn off” an inhibitor (for example,a sulfur-containing compound). Increasing the temperature of a sensingelement in a combustible gas sensor to, for example, burn off aninhibitor in response to an electronic interrogation of the sensingelement of the sensor is disclosed in U.S. patent application Ser. No.13/795,452, filed Mar. 12, 2013. Instead of automating theabove-identified maintenance procedures, a user may alternatively beprovided an indication of the need to perform any of the procedures.

The user may also be notified of an impending “end of life” of a sensor.For example, a user may be notified that the sensor should be replacedin “X” days or another time period. Likewise, the user may be notifiedof scheduled maintenance tasks required. For example, the user may benotified that a gas calibration is required in “X” days or another timeperiod. Pre-planned or scheduled maintenance may, for example, bealtered on the basis of the results of one or more interrogations.

The life and health test or electronic interrogation test may be run onmultiple sensors within the instrument. Such an electronic interrogationmay also be run upon a sensor (whether analytical or non-analytical)which is responsive to the driving force associated with the flow pathtest (for example, a non-analytical oxygen sensor) to test theoperational status or functionality of that sensor. The results ofelectronic interrogations of multiple sensors can be combined in ananalytical algorithm to determine actions (as, for example, describedabove) based upon that data. As described above, it is common forportable gas detection instruments to contain several sensors with aplenum through which gas is pumped by and external or internal gas pump.The sensors typically included in such an instrument would be acombustible gas sensor, an analytical oxygen sensor (which may or maynot include a non-analytical oxygen sensing element or electrode) andseveral toxic gas sensors such as carbon monoxide and hydrogen sulfidesensors. At least one of the toxic gas sensors may, for example, includea non-analytical oxygen sensor channel for performing a flow path testhereof. As described below, it is possible to monitor the current of thepump to determine the condition of flow through the plenum. In addition,under normal operating conditions, the output of the analytical oxygensensor should correspond to that expected for value of 20.8 vol-%(atmospheric) oxygen. Finally, the results of electronic interrogationof any or all of the electrochemical sensors, along with the results ofapplying a driving force to those sensors with non-analytical channelsintended to respond to such a driving force (that is a flow path test)may be combined together with, for example, pump current (and/or otherpump interrogation) measurements and the output signal of the analyticaloxygen sensor to give a high degree of reliability that all sensors inthe plenum are experiencing correct flow and are operating as intended.If, however, the results of these tests, either singly or taken togetherindicate a non-conforming condition, the combination of signals providea means of differentially informing the user of the nature of the nonconforming condition. For example, if the pump current is correct, butthe result of the flow path test (for example, applying a driving forceto which the non-analytical channel is sensitive) for a particularsensor is non-conforming, then that particular sensor requiresmaintenance. If however, the pump current is non-conforming, but theoutput of the analytical oxygen sensor is as expected, this wouldindicate a potential problem with the pump itself, or with is associateddriving circuitry.

Once “conforming” results are obtained in the embodiment of FIG. 14A,the users (or the system) may move onto an inlet/flow path blockagetesting as described herein. The system will enable the flow path testprocess and collect the associated data. The system then applies anyassociated algorithm/analysis and may, for example, compare the resultsto stored limits or thresholds. If the results are “non-Conforming,” anerror code or other indication may be provided to inform the user of anyrepair or replacement options (for example, replacement of filtersetc.). If the results are “conforming”, any and all associated sensoroutput corrections are applied. The user may also be informed that theinstrument is ready for use.

In addition to sensor output corrections associated with the electronicinterrogation of the sensor, the system may also apply one or morecorrections to sensor output determined as a result of the flow pathtest. In that regard, sensors may, for example, be thought of as“molecule counters”. Analytical sensors are thus calibrated in a mannerthat a certain amount of analyte molecules react at the analyticalworking or sensing electrode(s) as they diffuse through the instrumentand measured values are converted to, for example, a part per million(ppm) or percentage based equivalent readings based upon previouscalibration. When the inlet is open and unobstructed, rates of diffusionare very repeatable under the same conditions. As any instrument inletbecomes blocked or flow paths are otherwise obstructed, the rate atwhich the molecules can diffuse from outside the instrument housing tothe sensor can slow, thus lowering the rate at which molecules willencounter the active portion of the sensor (for example, the workingelectrode of an electrochemical sensor), and thereby lowering theoutput. By measuring partial blockages as a result of one or more flowpath tests hereof, one can adjust the sensitivity of the sensor tomaintain accurate readings regardless of such partial blockages.

In a number of embodiments hereof, once a flow path test such as anexhaled breath test is complete, the system calculates a derivative ofthe sensor response, based on the function:

${{Rate}\mspace{14mu}{of}\mspace{14mu}{change}} = {\frac{d\; x}{d\; t} = \frac{x_{({t + 1})} - x_{t}}{\left( {t + 1} \right) - t}}$

The equation shown above indicates a generalized derivative function. Asis known to one skilled in the art, there are many arithmetic formulaswhich can be used to calculate a derivate from periodic data.

FIG. 15A illustrates sensor response or output for a typical flow pathtest hereof in the form of an exhaled breath test as a function of time.FIG. 15B illustrates a plot of the rate of change of the sensor responseof FIG. 15A. A peak rate of change as percentage of the baseline is thencalculated as follows:

${{Peak}\mspace{14mu}{rate}\mspace{14mu}{of}\mspace{14mu}{change}} = \frac{{peak}_{\max}}{baseline}$

Referring to FIG. 15B, the peak_(max) corresponds to the maximum valueof the derivative function immediately subsequent to the application ofthe driving force, resulting in the positive deflection shown in FIG.15A, and the baseline refers to a mean value of the derivative functionprior to the application of the driving force.

The peak rate of change values may be correlated with a correctionfactor as illustrated in the plot of FIG. 15C. From the plot of FIG.15C, an associated lookup table or an associated algorithm/formula, thesystem may determine a correction factor for sensor sensitivity basedupon the calculated peak rate of change.

An embodiment of a control procedure and fault detection procedure for agas detection system or instrument that may be operated in a forced flowmode (that is, using a pneumatic pump to draw environmental gasses tothe one or more sensors of the instrument as described in connectionwith FIG. 3I) is illustrated in FIGS. 16A and 16B. The control procedureand fault protection procedure may be used in connection with a systemsuch as system 400 which may be operated in a forced flow mode as wellas in a diffusion mode (that is, relying on diffusion to bringenvironmental gasses to the one or more sensors of the instrument asillustrated in FIG. 3J). The illustrated procedure is discussed infurther detail in U.S. Pat. No. 6,092,992, the disclosure of which isincorporated herein by reference, and provides another independent checkof instrument or system operational state. Computer code for theprocedure may be stored in memory system 405 and is discussed inconnection with the pseudocode set forth in the appendix to thespecification of U.S. Pat. No. 6,092,992. Under the illustratedprocedure, when the power switch of the gas detection instrument orsystem such as system 410 is turned on, a pump initialization procedurebegins. A control system, which may, for example, include a processorsystem 404 (for example, including a microprocessor) in communicativeconnection with memory system 405) and/or control circuitry, preferablyfirst checks to see if motor 406 of pump 406 a is connected within theinstrument or system 410 by measuring if a motor signal (for example,back emf) is being generated. If no motor signal is detected, the pumpinitialization procedure is exited and the gas detection instrument maybe readily operated in a diffusion mode.

If motor 406 is detected, the duty cycle is set to 100% (percent on) forapproximately 0.5 seconds. Microcontroller/processor 404 measures thepower available from a power source such as a battery 408, and then setsthe duty cycle to a maximum duty cycle previously established for themeasured battery voltage. A maximum duty cycle and a minimum duty cyclefor given battery voltage ranges may, for example, be establishedexperimentally for a given pump and motor combination to provide anacceptable flow rate. For example, for the motor and pump combinationcontrolled via the pseudocode set forth in U.S. Pat. No. 6,092,992, amaximum duty cycle of 80% and a minimum duty cycle of 5% wereexperimentally established to provide an acceptable flow rate for abattery voltage of greater than approximately 3.6 volts. For a batteryvoltage equal to or between approximately 3.6 and 3.3 volts, the maximumand minimum duty cycles were experimentally determined to be 90% and 5%,respectively. For a battery voltage less than approximately 3.3 volts,the maximum and minimum duty cycles were experimentally determined to be100% and 5%, respectively.

In a number of embodiments, a PUMP CHECK procedure (see FIG. 16B) isinitiated after the duty cycle is set to the maximum duty cycle for themeasured battery voltage. The PUMP CHECK procedure first determines if apump has been added to the gas detection instrument since the instrumenthas been turned on. If the pump is newly added, a fault is preferablyindicated and the user is required to actuate a reset button to begininitialization of the newly added pump. Likewise, in a number ofembodiments, removal of a pump results in a fault indication requiringthe user to actuate the reset button to continue to operate theinstrument in the diffusion mode.

The PUMP CHECK procedure is exited if a fault condition has beendetected and a fault indication has been given. Upon initializationafter turning on the instrument, however, fault indications arepreferably delayed for up to 15 seconds for centering. If no faultcondition has been detected, the PUMP CHECK procedure determines if aPULSE CHECK procedure is in progress. During initialization, however,the PULSE CHECK procedure is disabled for a period of, for example, 30seconds in a number of embodiments. If no PULSE CHECK procedure is inprogress, processor 404 may, for example, attempt to adjust the dutycycle in a manner to achieve a motor signal (average back emf voltage)centered between a maximum acceptable average voltage and a minimumacceptable average voltage experimentally determined to efficientlyprovide an acceptable flow rate. For example, for the pump and motorcombination in the pseudocode of U.S. Pat. No. 6,092,992, the maximumand minimum motor signals were established to be approximately 1.95 and1.85 volts, respectively. Processor 404 thus attempts to adjust the dutycycle to achieve a motor signal of approximately 1.90 volts. A motorsignal in the range of approximately 1.85 to 1.95 volts may, forexample, be considered to be centered, however. If pump motor 10 is notcentered within, for example, 15 seconds, a pump fault is indicated byan electronic alarm system 90 such as an alarm light and/or an alarmsound.

If motor 10 is centered, the PUMP CHECK procedure checks whether it istime for a PULSE CHECK procedure. If yes, the PULSE CHECK procedure asdescribed above is initiated. If no, processor 404 checks for faults. Asdiscussed above, during operation of gas detection instrument or system400 the average back emf or motor signal may, for example, be centeredbetween 1.95 and 1.85 volts to maintain a suitable flow rate. Faultindications are enabled only when the motor signal is maintained in thisrange. If the duty cycle has been set to the minimum duty or the maximumduty for a defined period of time such as one second or more incontrolling motor 406, a fault is indicated. Moreover, if the motorsignal is less than approximately 1.4 volts for a defined period of timesuch as one second or more, a fault is indicated. Further, if the rateof change of the duty cycle is greater than 5% during, for example, afive second interval, a fault is indicated. Like the maximum and minimumduty cycles and the target motor signal range, the 1.4 volt minimummotor signal and 5%/5 second rate of change thresholds or faultconditions are readily determined experimentally for the pump and motorcombination in use. If no fault condition is identified, the PUMP CHECKprocedure is exited. After initialization, the PUMP CHECK procedure orfunction may, for example, be called or executed periodically (forexample, 10 times per second).

Any time a fault condition is identified, the duty cycle may, forexample, be set to its minimum duty cycle for the battery voltage. In anumber of embodiments, the PUMP CONTROL procedure checks the batteryvoltage periodically (for example, once per minute) to set theappropriate maximum and minimum duty cycles.

In the embodiment of the PULSE CHECK procedure set forth in thepseudocode of U.S. Pat. No. 6,092,992, microcontroller 404 determines ifthe average voltage across motor 10 is less than 1.4 volts after astart-up period of approximately 1.5 seconds if the temperature isgreater than or equal to 5° C. If the temperature is less than 5° C.,the determination is made after a period of approximately 2 seconds. Ifthe motor signal is less than 1.4 volts after the start-up period, afault is indicated. The start-up voltage threshold of 1.4 volts may bedetermined experimentally for a particular pump and motor combination.

Pump and motor combinations may, for example, be tested over a range ofload conditions, temperature conditions and battery voltages. Faultparameters or thresholds may, for example, be established by simulatingvarious fault conditions. Various fault detection systems and methodsmay be used collectively or individually to detect pumping faultconditions in gas detection instruments. Blockage may, for example, beperiodically simulated to test the continued operation of such systemsand methods.

FIG. 16C illustrates a system to effect control of pump 406 includingmotor 406 a which drives pump 406 as described above. In the illustratedembodiment, motor 406 a receives energy from a battery system 405 via aswitch mechanism such as a transistor switch using Pulse WidthModulation (PWM). In PWM, the battery voltage is generally pulsed on andoff hundreds of times per second. The time duration or duty cycle ofeach pulse is varied to control the speed of motor 406 a. While thetransistor switch is on, battery system 405 supplies power to motor 406a which energizes the windings of motor 406 a and causes motor 406 a toturn. While transistor switch is off, motor 406 a continues to turnbecause of its momentum and acts like a generator to produce backelectromotive force (emf). The energy (that is, the back emf) can beredirected back to motor 406 a using a regeneration circuit including,for example, one or more diodes connected across motor 406 a. Thistechnique is known as regeneration. The back emf can also be used toprovide feedback to control motor 406 a.

A motor signal proportional to a voltage across the windings of motor406 a while the transistor switch is in the off state is measured andused to control motor 406 a. There are a number of ways in which a motorsignal proportional to the voltage across the windings during the offportion of the PWM cycle can be measured. For example, the approximatevoltage at any defined instant during the off portion of each cycle canbe measured. Further, the approximate average voltage developed acrossmotor 406 a during the off portion of the PWM cycle can be measured. Ina number of embodiments, the approximate average voltage developedacross motor 406 a during both the off portion and the on portion of thePWM cycle is measured.

Each of the above measurements is proportional to the voltagecontributed by the regeneration phase of the cycle. The voltagecontributed by the regeneration phase is, in turn, proportional to thespeed of motor 406 a. Under light load conditions, motor 406 a runs at arelatively high speed and generates a high voltage. When the load onmotor 406 a increases, motor 406 a runs at a lower speed (assuming theenergizing pulse has not changed) and the voltage decreases. In a numberof embodiments, a microprocessor or microcontroller of processor system404 measures the voltage decrease and then increases the pulse width (orduty cycle) proportionally to compensate for the load until the motorvoltage is back to its normal operating value or within its normaloperating range. When the load is removed, motor 406 a will speed upmomentarily and increase the voltage. Processor system 404 adjusts theduty cycle until the voltage is again back to its normal operating valueor range.

By controlling the motor voltage, the speed of motor 406 a, and therebythe flow rate of pump 406, are maintained in a relatively smalloperating range. Efficient motor control maximizes the life of batterysystem 405. The normal operating conditions of motor 406 a under lightand heavy loads are preferably characterized to determine the maximumand minimum duty cycle required for motor 406 a over battery voltagechanges and operating temperature changes normally experienced duringuse thereof. These maximum and minimum values may be used to determinenormal operating limits for motor 406 a and to detect problems in theflow system such as a sample line failure or a motor failure. A cloggedsample line or a stalled motor condition, for example, is detected by alow average motor voltage. A burned out motor winding or an opencommutator circuit is detected by the absence of the regeneratedvoltage.

A system and a method for detecting more marginal fault conditions, forexample, caused by sudden changes in pneumatic loading may also beprovided. Such sudden changes may occur, for example, when a liquid isinadvertently drawn into the free end of the sample line or when thesample line is restricted by a crushing force somewhere along itslength. In one embodiment, the control system illustrated in FIG. 16Cmeasures the rate of change in the value of the PWM required to maintainthe average motor voltage constant. Once a predetermined center point orcontrol point of average motor voltage is obtained, processor system 404thereafter continuously adjusts the PWM to maintain the voltage constantand computes the rate of change in the PWM. The computed rate of changeis continuously compared to an empirically determined normal, acceptablevalue of rate of change and any deviation in the computed rate greaterthan this acceptable rate is interpreted by processor system as a flowsystem failure or fault condition.

In another embodiment processor system 404 causes a momentary shutdownof the PWM supply signal on a periodic basis and subsequently verifiesthe generation of an acceptable average motor voltage within a set timeinterval after the resumption of the PWM supply signal. This procedureis referred to as a PULSE CHECK procedure in connection with FIG. 16A.The periodic shutdown may, for example, occur approximately every 15seconds. This period is sufficiently frequent to monitor pump 406 andsample system performance, but not so frequent as to materially reducethe effective sample flow rate. The PWM shutdown period in thisembodiment may, for example, be approximately 0.2 second. This shutdownperiod is sufficiently long to cause motor 406 a to stall (that is, toslow or stop) and to allow the checking of the acceleration of motor 406a upon resumption of PWM within a predetermined interval of time. In anumber of embodiments, the interval chosen for motor 406 to accelerateto a defined average voltage was approximately 1.5 seconds after theresumption of the PWM supply signal. While 1.5 seconds is an appropriatevalue around room temperatures, at lower temperatures more time may beallowed because of the slower acceleration of motor 406 arising from the“stiffness” of the mechanical components of pump 406 at such lowertemperatures. Absent a marginal fault, motor 406 a will restartsuccessfully (that is, within the defined time interval after theresumption of the PWM motor 406 a will again be regenerating anacceptable average voltage). A failure to “successfully” restartindicates a fault condition. For example, a marginal fault conditioncausing an excessive demand for motor torque upon restart is detected asa lower than normal average voltage at the end of the time interval andis interpreted by processor system 404 as a flow system failure. Testingthe demand pump 406 for motor torque at a predetermined PWM provides avaluable check for a number of fault conditions.

The foregoing description and accompanying drawings set forthrepresentative embodiments at the present time. Various modifications,additions and alternative designs will, of course, become apparent tothose skilled in the art in light of the foregoing teachings withoutdeparting from the scope hereof, which is indicated by the followingclaims rather than by the foregoing description. All changes andvariations that fall within the meaning and range of equivalency of theclaims are to be embraced within their scope.

What is claimed is:
 1. A method of operating a system including at leasta first sensor to detect a first gas analyte, the first sensor beingpositioned within a housing, the housing having at least one inlet, anda control system, the method comprising: testing a state of at least onetransport path of the system by creating a driving force in the vicinityof the at least one inlet of the housing other than by application of acalibration gas including the first gas analyte or a simulant gas forthe first gas analyte, measuring a non-analytical response of a second,non-analytical sensor to the driving force and adjusting an output ofthe at least first sensor via the control system at least in part on thebasis of the results of testing the state of the at least one transportpath.
 2. The method of claim 1 comprising measuring the rate of changeof response of the second, non-analytical sensor to the driving forceand correlating the rate of change of response of the second,non-analytical sensor to a correction factor for sensitivity of thefirst sensor to the gas analyte.
 3. The method of claim 2 wherein a peakin the rate of change of response of the second, non-analytical sensorto the driving force is correlated to a correction factor forsensitivity of the first sensor to the gas analyte.
 4. The method ofclaim 1 wherein the first sensor is an electrochemical sensor.